I copy all files onto a USB stick for myself, son and brother and we all watch them on the PS3's. It's the easiest way I've found as you don't have any juddering, skipping, etc. Just takes a few minutes to copy onto the USB and plug in.

I'm pretty sure if you google this problem with mkv files (Blu Ray iirc) you should find a solution on how to stream them. If they are 1080p avi files then also google search this to find a solution.

As for Mac streaming to the PS3 you need something called "EyeConnect" which I use