Got a red area on inner thighs after sitting for long hours, it became small red dots after using beta-micoter for 3 weeks, should i use lamisil (terbinafine)?

Rash: The rash did not respond to topical steroids for 3 weeks, it will be prudent to have a doctor look at it and recommend best treatment.

Fungi: It could be a fungal infection which can be made worse with heat. I would try Lotrisone (clotrimazole and betamethasone) cream which may decrease the inflammation and if it is fungal will take care of that.