On average, how fast do kidney stones grow/form? 3/11/21 us and ct showed 1 barely noticeable stone. ct with contrast 3/25/21 showed three 2-3mm nonobstructing stones?

Stones grow at a variable rate depending on many things. stones don't grow from barely visible to 2-3 mm in 4days. Chances are that technique or position might have made the difference.

US is not reliable for detecting small stone. Because the stones are quite small so it is not uncommon some were missed with the first CT. Unless it is absolutely indicated, CT should not be done too frequent because you will be exposed to a lot of radiation. To monitor kidney stone, US can be used and it CT needs to be done, low radiation technique should be used.