3 rogues + different specs = trouble

considering how positional some specs of the rogue class are, you will have a really hard time getting one rogue in position to garrote/backstab while the others are whacking away.

can X spec computer hand Y instances of WoW? - how long is a piece of string? - trial accounts are free, just set it up and see if you get decent frame rates! seriously! you don't have to spend days doing macros to see if your machine can handle the FPS you need/want. just log in, create a few toons of the same race, and see what happens when you zone in. you don't even need to move around, but you can if you like. If you REALLY want to test it, run the group to a major city like Stormwind. For this, keyclone/octopus isn't even necesary - just use "/follow LEADTOONNAME" and let them follow you to the city.