The important thing is getting the outputs balanced so that the in-between positions are getting the right proportion of signal from each pickup - that's why so many people sell Strat sets, and why it's easy to mess up a "classic" setup by sticking some 16K mongo-overdrive pickup in with the regular ones. I've read of a "10% Rule" - the middle is supposed to be 10% more powerful than the neck, the bridge is is supposed to be 10% more powerful than the middle - but I don't know if it true. Mixing pickups from different manufacturers is always a crapshoot, as they each measure output via voodoo magic and entrail divination... it'll work O.K. eventually but be be prepared to spend some time trading pickups with your friends & neighbors.
I personally don't care for "strat cluck" but if it's important to you quite a few people put mini trimpots under the pickguard and balance the pickup outputs that way. It's real common among the Nashville studio guys with three-pickup Teles, who can't/won't put extra pots on top.
Three concentric pots would give you six different knobs to play with.... :blob7: