I want to make sure I clearly understand what you're planning. From what I read, you're going to drain the top one down to the next one, etc. If you don't have some type of auto-siphon, such as a bell siphon, or the ones pictured by JeffW or Wolfracer, the water will just drain straight through, instead of rising to within 1/2" to 1" of the top of the grow-bed to provide the plants with the necessary water and nutrients - mine actually goes over the top of the grow-bed, because I use cups to grow in. I'm assuming, of course, that this is planned to be a Flood and Drain System. One of these days, I'll get around to putting up some photos of my no-holes siphons so you can see how they work ..... as a matter of fact, I'll see if I can break away from the FAQS that I'm working on, and put up a couple of pics of my system (now that it's mostly recovered from my killing spree) so you can see how the no-holes system works. I think my grow-beds are about the same size as the trays you are getting. A three inch drain seems a little extreme unless you have a huge pump. I think my pump is a 350GPH with a 1/2" line providing the water to the grow-beds, I have been using a 1" drain until recently when I've been trying out a different no-holes drain, which added a 3/4" drain, so the two are running side-by-side, but I still get plenty of water in the lower grow-beds.
Jackalope