I like to use headers for programming, designed into the circuit from the beginning. you could have a set of headers and use a interface board between your programmers and regular board. You could use 1 programmer and use bus switches to determine which chip is programmed. a simple 628 chip with a button select and a couple leds could drive the bus switches.

Also if you dont want to do this and still want to use 3 programmers, then any of the pins tied togeter use a jumper, remove jumper to program.