How do I stop an LED Troffer flickering problem?
So, your problem is that you are wiring lights in series.

This is not how you wire lights.

Single gets full voltage, works fine

Two in series, half voltage, apparently still works.

3 in series, 1/3 voltage, no worky.

4,5,6 in series, 1/4, 1/5, 1/6 voltage, no worky worse.

You are outing yourself as perhaps not actually qualified to be replacing lights, if you don't know (and evidently did not research) to wire them in parallel, as they are meant to be, so that each light gets full voltage as intended.