3x 300l unvented cylinders in a basement.

[DP]

What about adding solenoids to cyl cold feeds,link solenoids via high limit float switch. Or just single solenoid depending on main incoming pipe size

Bad idea that.
Imagine water keeps getting heated in the cylinder, reaches 95 degrees C, T&P valve lifts. With your setup, no incoming cold water to cool the cylinder that above valve would have effected. One water starts to boil in the cylinder wonder how what the outcome would be