Welcome Rif!

I will try to give my thoughts on each piece of gear you asked about.

Sleeping Bag: Wow! that is a monster of a bag... rated to -40F! Three-season for the Sierra Nevada in Cal people usually get around a 20F bag (or quilt). If you are a cold sleeper may be 10F or 0F. If you are a warm sleeper maybe a 30F or 40F if you really like to push things. Your bag is arctic expedition level. You can get a quilt for less than a third the price and a third the weight. I have an Enlightened Equipment quilt: https://enlightenedequipment.com/revelation/

If you really like bags the WM Ultralite might more closely match your needs: http://www.westernmountaineering.com/sleeping-bags/extremelite-series/ultralite/

If you really need a bag rated to -40F then you can't get much better than the one you have.

Backpack: If you need the Bison bag than you are going to need a big backpack like you got. It seems like a pretty good choice. For 3 nights in more reasonable conditions (down to freezing) I think it is too much. The standard advice around here is to get the backpack last to match your gear... but I actually advise people to get the backpack to match there trip and that will let them know if they are bringing too much gear. For 3-season/3-night trip a 50 - 60L backpack should be more than enough.

Your buddies recommended a waterproof backpack. I haven't found much utility in them. I find it is lighter to protect stuff (sleeping bag and clothes) inside your pack with a trash compactor bag. If it wears out you can just get another one. If the waterproofing on your backpack wears out you have to buy a new backpack. If I expect significant rain I bring a poncho that covers my backpack.

The most important thing about a backpack is how it fits you with your gear. Is that Gregory really comfortable on you with your gear?

Tent: That sounds more a campground tent than a backpacking tent. You can get 2P tents in the 2-3 lb range. Your buddies recommend a bivy. They are bit too squashed for me particularly if you are dealing with rain. It is nice to have somewhere to get of the weather where you can sit up. Often people match bivies with a tarp and they don't end up much lighter than a 1P tent.

Edited by BZH (12/06/18 12:09 PM)
Edit Reason: changed WM recommended bag from Alpinlite (wide) to Ultralite (reg)