What do you think would be the best way to try and make 3 5-gallon batches and have them as consistent as possible? I am brewing for a party and want all 3 kegs to be so similar that you wouldn't be able to tell that they weren't a single batch.
You didn't say whether you intend to make them serially or consecutively (edit: oops, I meant concurrently). If you can't make one 15-gallon batch, or blend all three batches together after they're done (very gently, and without unduly exposing them to air) before kegging, temperature control is probably where you want to concentrate. I suggest you keep them all in the same room or the same cupboard—we do what we can, right?—and at the same temperature.