How can I provide neutral wires for my smart switches?
On the hots, you can add the switch hots via pigtail to that nut with the black wires.  

Yes, when all the wires in the bundle are white, that is neutral.  Do Not split up the neutral bundle, those are not spare, those are how current returns.  The only reason you're thinking that is because you don't have any pigtail wires or wire-nuts on hand and you can't see how to connect without those things.  Go get them!

You need to add your neutral wire to the bundle.  Since that bundle is pretty full already and I gather you will have 3 smart switches, I recommend adding a white pigtail to that bundle and joining the switch wires to the pigtail with another wire nut.