Most girls won't admit it but they really enjoy their girlfriend inserting a dildo or a vibrator gently but firmly into the anus. Some girls like a large one and others a smaller one, but the result is the same. Three girls together is even better, one egging the other on while the lucky recipient enjoys the feeling, then swapping so they all do.
I have explored the idea of anal insertions in the past but it always bears repetition. Enjoy.
And just look at that sexy split thong.

You can see how much she loves it.

This girl takes a smaller vibrator but she loves it just as much.