3 months out from under eye fat injections, can some of this extra fat be sucked out?

It's very difficult to remove grafted fat in the face.
This is especially true in thin areas like under the eyes.
Some people may recommend steroid injections. the anti inflammatory steroids can as a side effect cause fat atrophy(shrinking).
This is a fairly rare complication and when it happens can lead to substantial hollowing.
Probably the most effective option is direct surgical excision.
The skin in the lower eyelid area tends to heal very well with minimal scarring for the most part.
This is an unfortunate fairly common occurrence. 
You are the second patient this morning was posted a question with the same concern.
Many plastic surgeons are reluctant about grafting fat in this area.
Best,
Mats Hagstrom, MD