That’s up for debate. Fred would like all 3 phases switched together, to apply the full 3 kW load to all 3 phases equally. In that case, the wear on the elements will also be equal. But as I pointed out earlier, if less than 3 kW of PV is available, it’s possible, by connecting just two phases, to have two heating loads in series to give 1.5 kW shared over two phases - and if those phases can be chosen to be the most lightly loaded pair, then it will likely improve the balance between phases.
I would guess that the 3-phase heater is actually 3 separate 230 V, 1 kW units, just wired in star, so replacing one should not be a problem.
Neither do I, nor most domestic consumers in the UK! Here, only very few houses are not on a single phase, 240 V supply. A 3 kW immersion heater is common here, drawing 12.5 A. So I don’t really understand the concerns with an out-of balance current of just 4.3 A.
[Ignore the "Test response … " link, something funny happened between Discourse and my browser.]