First, a peat pellet is too small to start one those big seeds. by the time a zucchini seeds pushing up it's seed leaves, it would have grown a root that is probably twice as deep as what the pellet puff up to.
The fact that you said you started them in the "greenhouse" -- i.e. With their covers tells me the light source was most likely too far away, that and the fact the you said youn"staked" the seedling before first leaves grew.
Starting large cucurbits in small containers is an iffy proposition to begin with, but it should be done in a 6" deep container IMHO. I would use a 1qt deli or dairy -- yogurt, ricotta, etc -- container with PLENTY of holes.
Cucurbits hate their roots to be constricted or disturbed.
When you planted the pellets with the seedling, did you remove the netting? what did the roots look like? also did you intend to grow them in the 3 gal containers because that would be way too small....
You can't give them enough light or heat. Too much would only happen if you let the leaves touch the light bulbs/tubes, then the leaves will get scorched.
I'm really sorry, I feel like I'm bashing you at every step, and I don't mean to.
It's simply easiest to sow zucchini seeds directly in the ground in the sunniest location you can.
. They'll sprout and grow quickly once the soil has warmed up. Watch out for SVBs (squash vine borers) and Squash Bugs.