A robot party - what a wonderfully unique idea!! LOL, I love it.

I think if we're talking mostly 3yo's at the party then you're better off keeping it simple. If it were me, I would incorporate a few of the great ideas already posted above, but make the 'focus' of the party the robot costumes/boxes. Rather than setting up specific activities, I would tend to just lay out heaps of hula-hoops, skipping ropes etc on the ground so they can jump over them, run around and just be kids! They'll probably be so excited to be getting around as robots they won't have the attention span to follow any games or activities more complicated than that!!!