With no neutral connection, you will only need to switch two of the phases, you can leave the third phase connected (only switched by an isolating switch and circuit breaker for protection). If you switch one other phase on, you will have a reduced load of 1.5 kW spread equally over the permanently connected phase and the one that is switched on. This means that you can have three stages of loading: zero, 1.5 kW or 3 kW. It is not necessary to load the three phases equally: when you are using other things, I can almost guarantee that the three phases will not be loaded equally. But your power company would like it if you did - especially if that was your total house use and not just the immersion heater.
If you and Robin between you decide to switch all three phases and there is no neutral connection, then you have a choice of: zero, 1.5 kW spread equally over your choice of any two phases, or 3 kW spread equally over all three phases.
If you have the neutral connection, then you must switch all three phases. Then you have a choice of: zero, 1 kW on one chosen phase, 2×1 kW on your choice of 2 phases, or 3 kW spread equally over all three phases.
If you have a neutral connection and you can switch it but leave one phase permanently connected (I don’t know if that’s possible with Robin’s PCB design), then you have a choice of: zero, 1 kW on the permanently connected phase, 1.5 kW spread equally over your choice of that permanently connected phase and one other, 2×1 kW on your choice of that permanently connected phase and one other, or 3 kW spread equally over all three phases.
Each of these might allow you to achieve a better overall balance for the three phases for part of the time; also you might be able to reduce flicker, because instead of switching 4.3 A on each phase simultaneously, you would be switching between zero and 3.75 A, or between 3.75 A and 4.3 A, or between zero and 4.3 A on one or two of the three phases.
That is indeed correct, but does it matter? I would say it is good when that happens, but it is not essential with loads of only 4.3 A per phase.
If you connect two heating elements in series (with no connection to neutral), then you will be putting a 860 VA reactive load on the two phases, inductive on one and capacitive on the other. The inductive load will cause the voltage to drop slightly, the capacitive load might cause the voltage to rise slightly. But that should not cause a serious problem.