A queen bed (mattress) is 60" wide. Euro shams are usually 24"-26" square. Stuffing them tightly will cause some take-up, but they'll look like marshmallows. You could overlap them, but I don't think that will give you the look you're after.

Yes. And you do not need to "stuff very full" Buy a down/feather blend insert in 26 x 26 and one sham will sit slightly in front of the back two, No issue and a nice look.

You don't need them as hard as a brick bat in filling. Not a good look anyway : (

I would only use two euro shams on a queen. I would use three on a king size. If you use three on a queen, the pillows are going to dwarf the bed and make it look too small.

A standard euro sham is 26" x 26", therefore, two are enough on a queen bed.

I have a queen sized bed and my comforter set came with two queen sized pillow shams. When I positioned both pillows up against the headboard, I do have enough room to put a third pillow in the middle, since the middles do not meet. I prefer that look to having a bunch of overstuffed pillows.