Doing it the "naive" way would be: for every object that needs to be rendered like this, loop through all the lights and find the closest 3 of them. That would be O(n * m) where n is the number of lights and m is the number of objects. If you have hundreds of lights and thousands of objects, it's not insignificant, especially since you're doing it on the main thread.

I had to do this for a Unity game which had a similar "custom" light solution, and on lower end devices it was a problem. It was easily solved using a simple spacial data structure, but doing it the naive way did present some issues.