As far as shifting preference, I've ridden both twist-shift and rapidfire and greatly prefer rapidfire. The ability to shift without moving you hand position is nice when standing or braking simutaneously. I rode twist-shift for 5 years and just recently switched to rapidfire. Wow, what a difference.
As far as bikes and prices, it's doubtful that you're going to get a break on the price of a new bike. Bikes are very low margin so the shop doesn't make a lot of money on them anyway. Most bikes are priced just over what the shop paid for them so there's not going to be a lot of wiggle room.
Where you might catch a break is on accessories. That's where shops make a lot of money so they're more willing to flex on pricing. If you're buying three bikes, you may be able to get a deal on all the other stuff you're going to need or want, like helmets, pumps, tools, racks, camelbacks, etc.