The one nearest the switch still comes on, but not the next two.
It's not the tubes, because I tested all three in the one that works, and they all come on.
Sometime ago I noticed a very slight, intermittent popping noise -- almost like a water-drip -- in the area of the second on the line.
I notice that when I take the tubes out of the non-functioning holders, the ends of the tubes are quite warm.
I've tested all circuit-breakers.
Any guidance appreciated.