Can i combine tretinoin, benzoyl peroxide and topical erythromycin for adult acne? if yes, in what sequence?

Yes but drying.: I would try benzoyl peroxide followed by retin a (tretinoin) then Erythromycin but you could switch the order of the latter two. Of course other physicians may recommend other regimens. Your skin will likely become quite dry and may benefit from gentle exfoliation first and the use of non comedogenic moisturizers in the am with sunscreen.

Yes: BP first then erythromycin.