Will this bag hold a 3 person tent, sleeping...
Will this bag hold a 3 person tent, sleeping bad and pad and enough clothes for 2-3 days?
No, I would recommend at least 3200cu in to fit that gear for 3 days. I imagine you have 1-2 other people going with you since you have a 3-person tent, so if you split up the tent and other gear, you will be able to fit everything you need in 3200+ cu in.
If you are going to be the only one carrying the tent and all your gear, I'd say step up to a 3400+ cu in backpack.
I agree, I can barely fit a 2p tent, 850 fill 30 degree bag, neo air, and necessary gear in a 55L bag.
This bag would not be big enough to put all the stuff you listed into. To me, its more of a day pack than for extended use.