mixing three cameras in real time

This depends entirely on what method (or objects rather) you are using to mix down the three sources. the inlets of jitter objects dont mix incoming matrices on their own, unlike MSP objects. sending multiple streams to a single inlet will cause this flicker, as the jit.pwindow object is simply displaying the last matrix it recieved. however I am guessing you already knew this and are using jit.op or jit.xfade to mix the streams? in which case flicker could be coming from a timing issue, a lack of sync between your sources.

the easiest way to deal with this would be to create 3 jit.matrix objects to serve as buffers, which can be driven with a metro to control the timing.

also check out the jitter tutorial on 4 way slab mixing, it works pretty well...