anyone noticed that heater have internal thermostat that switches them off, while the

[mike31154]

Are all the heaters the same size/wattage? You mention they are different brands. Most are built pretty much the same with a simple bimetallic strip that switches them on/off. Dissimilar metals fused together bend at a different rate in relation to temperature. A set of contacts at the end make or break in response to the bending, completing or opening the circuit to the heating element.

Many things come in to play that influence the behaviour of the the bimetallic switch. Heater location (in relation to flow), size (both physical & wattage), variations/quality in manufacturing, water level in relation to the bimetallic strip, age of the heater, etc. Over time, the contacts become pitted & fail due to arcing every time they open/close. Pitting will increase contact resistance & alter the cycling rate of the heater, worst case they fuse together & the heater stays on as described by many folks who've had them fail that way.

Not sure having all three connected to a single channel on your controller is a good idea either. You've taken the redundancy of having 3 heaters & connected them to a single point of potential failure. Also make sure that the single channel you have them on is rated to handle the wattage of the 3 heaters. I assume it can, since it appears you've been running that way for a while, but never hurts to double check, since as mentioned, as contacts wear resistance increases and with it the actual power draw of each heater.