i have a 3 cam setup for a student project. i synched all the cams to the one audio from our single zoom via autosync in avid and then synced them all to have three video tracks (avid popup asking me if i wanted to group them). now, with my grouped video subclip, i mark it with 'T' and then overwrite it into a sequence. so that i may show the dailies to others in my group, i used blend > picture-in-picture to split the screen into 4 quadrants and fill 3 of them with cam's A, B, and C. that way, they can view all cams without the need to open more files. however, when all three effects are on the timeline, i get slow playback in the record monitor. it catches up after a moment. i am exporting now to see if it plagues the .mov files.
my specs are as follows
intel core 2 quad q6600 running at 3.2ghz
8gb of ram (4x2GB)
avid Media composer 6.5.2
The only suggestions I would have is to make sure you are not AMA linked and to try rendering the effects. You can always leave the effects off the timeline and go into multicam mode to see all 3 cams in the source monitor