Single pole to three lights
It sounds like a led problem to me also, although LED’s draw very little current arcing on the circuit can cause other lights on the same branch circuit to have problems. I would check the connections at that first fixture, worst case swap the lamp from the first and the 2nd or 3rd fixture a lamp with a problem might cause problems similar to bad connections. Give that a try. With LED’s I have have lots of problems with lamps and fixtures from overseas. I found out about DLC listing and have not had any flickering problems with them and the few that have failed were promptly replaced 2 different vendors. DLC is design light consortium, they require A 5 year warranty on most fixtures, they cost a bit more but I have installed over 300+ with only a few failures , non DLC lamps I have had close to 100% failures within 1 year and the warranty did not cover anything longer than 90 days. So it could be the lamp or the wiring both are possible, swapping them may fix the problem or verification of the wiring.