I've ridden with my 705 in a few downpours without problems, though not any 3+ hour ones, so I can't answer your question directly. I have owned wristwatches in the past where the manufacturer warned against pressing any of the buttons while the watch was wet. Apparently pressing a button breaks the seal in some case designs, but I don't know if this applies to the Garmin Edge (I hope not, that would be lame).
One thing I will say though, is that, IMHO, the memory card cover on the 705 seems like a huge water entry weak spot to me. The latch isn't very secure and I've had mine pop off a couple of times without me realizing it, including one time when I rode with it at least once before noticing the cover was missing (fortunately it wasn't raining, and I was able to find the cover later). It usually comes off when I have the unit in a pocket or bag, and probably accidentally press down on the latch when pulling the Edge out. I now keep some electrical tape over the cover (waterproof tape would be even better).