There will be a demo for Resistance 3, of the boat level that Insomniac Games already have showcased. Initially, the demo won't be available for grabs - instead it will come bundled with the action movie Battle L.A., when it is released on Blu-ray on June 14 in the US.
The Resistance 3 demo might show up on PSN as well, of course, but if you want it as soon as possible you'll need to pick up the movie that currently has the awesome grade of 34% on Rotten Tomatoes.