validate user using firebase auth

I want that the users those who have signed up in user app cannot sign into the admin or doctors app.
Or the doctors those who have signed up in doctors app cannot sign into user or admin app.
Or the admin those who have signed up in admin app cannot sign into doctors or users app.

The method i tried for user validation is

private void updateUI(final FirebaseUser user) {
    if (user!=null){

        final DatabaseReference adminsRef = FirebaseDatabase.getInstance().getReference().child("Admins").child(user.getUid());
        final DatabaseReference usersRef = FirebaseDatabase.getInstance().getReference().child("Users").child(user.getUid());
        adminsRef.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                if (dataSnapshot.exists()){

                    userRole = dataSnapshot.child("role").getValue(String.class);
                    if (user.equals("admin")){
                        FancyToast.makeText(getApplicationContext(),"User already registered as an Admin",FancyToast.LENGTH_LONG,FancyToast.INFO,false).show();
                        FirebaseAuth.getInstance().signOut();
                        if (AccessToken.getCurrentAccessToken() != null) {
                            LoginManager.getInstance().logOut();
                        }
                        startActivity(new Intent(MainActivity.this,MainActivity.class));
                        finish();
                    }
                    else {
                        FirebaseDatabase database = FirebaseDatabase.getInstance();
                        DatabaseReference userRef = database.getReference("Doctors").child(FirebaseAuth.getInstance().getCurrentUser().getUid());
                        HashMap<String, Object> map = new HashMap<>();
                        map.put("uid", FirebaseAuth.getInstance().getCurrentUser().getUid());
                        map.put("email", FirebaseAuth.getInstance().getCurrentUser().getEmail());
                        map.put("name", FirebaseAuth.getInstance().getCurrentUser().getDisplayName());
                        map.put("profile_status","incomplete");
                        map.put("role","doctor");
                        //map.put("location", getIntent().getExtras().get("location").toString());
                        userRef.updateChildren(map).addOnCompleteListener(new OnCompleteListener<Void>() {
                            @Override
                            public void onComplete(@NonNull Task<Void> task) {
                                if (task.isSuccessful()) {
                                    FancyToast.makeText(getApplicationContext(), "User details updated successfully", FancyToast.LENGTH_LONG, FancyToast.SUCCESS, true).show();
                                } else {
                                    FancyToast.makeText(getApplicationContext(), "User details update unsuccessful", FancyToast.LENGTH_LONG, FancyToast.SUCCESS, true).show();
                                }
                            }
                        });
                        Intent intent = new Intent(MainActivity.this, HomeActivity.class);
                        startActivity(intent);
                        finish();
                    }

                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
        usersRef.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                if (dataSnapshot.exists()){
                    userRole = dataSnapshot.child("role").getValue(String.class);
                    if (userRole.equals("user")){
                        FancyToast.makeText(getApplicationContext(),"User already registered as a Customer",FancyToast.LENGTH_LONG,FancyToast.INFO,false).show();
                        FirebaseAuth.getInstance().signOut();
                        if (AccessToken.getCurrentAccessToken() != null) {
                            LoginManager.getInstance().logOut();
                        }
                        startActivity(new Intent(MainActivity.this,MainActivity.class));
                        finish();
                    }
                    else {
                        FirebaseDatabase database = FirebaseDatabase.getInstance();
                        DatabaseReference userRef = database.getReference("Doctors").child(FirebaseAuth.getInstance().getCurrentUser().getUid());
                        HashMap<String, Object> map = new HashMap<>();
                        map.put("uid", FirebaseAuth.getInstance().getCurrentUser().getUid());
                        map.put("email", FirebaseAuth.getInstance().getCurrentUser().getEmail());
                        map.put("name", FirebaseAuth.getInstance().getCurrentUser().getDisplayName());
                        map.put("profile_status","incomplete");
                        map.put("role","doctor");
                        //map.put("location", getIntent().getExtras().get("location").toString());
                        userRef.updateChildren(map).addOnCompleteListener(new OnCompleteListener<Void>() {
                            @Override
                            public void onComplete(@NonNull Task<Void> task) {
                                if (task.isSuccessful()) {
                                    FancyToast.makeText(getApplicationContext(), "User details updated successfully", FancyToast.LENGTH_LONG, FancyToast.SUCCESS, true).show();
                                } else {
                                    FancyToast.makeText(getApplicationContext(), "User details update unsuccessful", FancyToast.LENGTH_LONG, FancyToast.SUCCESS, true).show();
                                }
                            }
                        });
                        Intent intent = new Intent(MainActivity.this, HomeActivity.class);
                        startActivity(intent);
                        finish();
                    }

                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });



    }

}

You are doing almost what needs to be done.
Just check if the user UID exists on Users child node or in admin child node using Data snapshot.

Here is how you can fetch the user role:

String userRole;

roleRef = FirebaseDatabase.getInstance().getReference().child("Users").child(firebaseAuth.getCurrentUser().getUid());
        roleRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                try {
                    userRole = dataSnapshot.child("role").getValue().toString();
                    userProf_username.setText(userNameInData);
                }catch (Throwable e){
                    Toast.makeText(getApplicationContext(), e.toString(), Toast.LENGTH_SHORT).show();
                }

            }

Now the role of user is in the string userRole. So now you can check if user is admin or normal client.

Then add an if-else block:

if(userRole.equals("admin")){
startActivity(new Intent(MainActivity.this, adminActivity.class);
}
else{
startActivity(new Intent(MainActivity.this, userActivity.class);
}