Can I use betnovate scalp application cream 3 weeks after FUE to reduce redness?

It is best to ask your surgeon about post operative care as each doctor has their unique regimen.  

Hydrocortisone cream ( or you may use betnovate cream which contains Betamethasone)are all  good for both itching and pinkish discoloration . But I recommend you to use additionally  Dermovate Scalp Lotion (Clobetasol Propionate) 0.05% 2 times a day that is a very strong (super-high-potency) corticosteroid against itching and redness.

You should check with your surgeon about how best to treat the redness that you are presently presenting with