I need to bake 3 large pans of lasagna in our conventional regular size oven. The pans are "13 x 9 x3" in size so I will need to use two oven racks.
Stack them, like this:
Rotate them 1/3 and 2/3 the way through cooking, figuring perhaps 25% longer cooking time than each would call for separately. Use an instant read thermometer, look for about 165F (74C). Also look for a bit of browning and bubbliness. It may be that they won't be done at exactly the same time, but they'll be close enough that they can be served at the same time.
I figure you might as well stack all three, and remove a shelf from the oven, but to do two and one (rotating each third the way through baking time) would accomplish pretty much the same thing.
If you can fit 2 pans on one shelf and one on another (like normal), that would be fine too, but you might have to rotate even more often. Swap shelves and front to back.