First of all, dont TELL girl A how you feel about her...SHOW her how you feel. be respectful, but not a puppy dog. I dont think you "love" her, I think you might be in love with the idea of being in love, otherwise you would have made your feelings known before now.
My personal advice is: If you value and respect girl A, dont sleep with girl B...she will find out and either be disgusted by your actions or (This second part is only if she secretly harbors feelings for you as well) she will be hurt by feelings of betrayal...either way...it will end badly. I think you should let girl A know how you feel (by your actions) and gauge her responces each time you go the extra mile. Dont be pushy, in fact most likely, if you can show her you care for her more than like friends, but at the same time let her know you wont be crushed if she doesnt feel the same way (that you can still be friends no mater what), then she might just approach you about taking your friendship to the next level.
Basically, let her know she is in charge, and whatever she feels you will respect...and take it from there.
If it doesnt work out...find a girl C to bang the stuffing out of!