Try using 4" 3-fletch with feathers instead. You'll wind up with practically the same fletching surface area as you'd get with 4 3" feathers and save some money in the process. Of course, the upside of using 4-fletch is not having to be careful about which way you nock them, and some folks feel that's enough benefit to offset the extra cost in time, feathers and money.

If you NEED the fletching area provided by 3 5" feathers to get your broadheads to fly straight, you'd be better off to go to 4" 4-fletch instead. Rather than giving up surface area, you'd actually be gaining some for even better control.

Speed? At the bow, you won't see any difference. Downrange though... The more surface area, the greater the drag, so you will see some changes in sight settings, but probably not until you're shooting beyond 40 yards. A light arrow will lose a greater percentage of it's downrange speed to drag than a heavier one will. Of course, if you wind up with a 4" 3-fletch or 3" 4-fletch instead of a 5" 3-fletch, you'll get somewhat faster arrow speeds downrange, but never trade off control and accuracy for speed. If your arrows fly great with less fletching though... and you're positive they'll do the same in actual hunting conditions... Go for it!

I'm afraid I can't answer your questions about rate of spin. It stands to reason that a 3" feather fletch will spin slower than a 5", assuming identical offset/helical settings, due to less surface area, but that's just 'thinking out loud' instead of knowing it for a fact.