Run heater on two powerlines, where the second line is switched according to the main line
From photo it is seen that each relay has input and output connection, therfore a simple rearangemnt of wires can drive a heater in a star connection with neutral in the middle. This is a part of yours PCB schematics and proper connection to 3ph network 230/400V:







simulate this circuit &ndash; Schematic created using CircuitLab

EDIT:
ad 1st) Use 5 wires of 2.5mm^2 cross section. Don't mix the colours, use standard CE colours: black for phase 3 wires, blue for neutral (or you might consider buying a cable 5x2.5mm^2 in this case you get black, brown, grey for phase, blue, yellow/green), yellow/green for earth. Install an overcurrent protection device 3x16A in your main cabinet. Now what you need is L1, L2, L3, N, PE wires (five of them).
ad 2nd) Phase sequence is not important, you can mix them.
ad 3rd) See my schematics: If you measure from N to any element you get resistance of one element R, if you measure between two of them you get a series resistance of two of them 2R. You got 28 and 50ohm which is close to R and 2R.
ad 4th) The free connections are one end from each element (red wires on your heater), while the second ends are connected together in star - the jumper you see that connect three heater ends, where the neutral is attached (black wire).