Would taking tums (calcium carbonate) before going to bed (without food) cause kidney stones to form? i've done this 3-times over a 2-week duration & got a 3mm stone.

Stones: No. Not likely causative for kidney stones. The risk factors for most types of kidney stones have little to do with calcium consumption. Especially short term as you've described.

Stone: Is remotely possible that Tums (calcium carbonate) would promote kidney stone, but it is likely there are other reasons, such as dehydration, oxalate excess, citrate deficiency, or a genetic problem. Requires evaluation.