Electronic – China single and 3 phase

I don't think that you will be happy with a regular power supply aproach, e.g. adjustable from 10V to 40V. For 40V and a regulation you need about 42V before the regulation. When you put it to 24V and draw 1000A you will have (42V - 24V) * 1000A = 18KW waste power.

You could mitigate this situation using a transformer with different output voltages for 12V, 24V and 36V. Already taken this step i would also consider if your starter motors realy need a regulated power supply, this would reduce your circuit to a transformer, rectifier and some capacitors. As a last step i would build a separate power supply for each voltage, then you could do some tests in parallel.

If you realy need a regulated power supply i would think about a switching one and considering the impression that i have from your experience in relation to the requirements i would perhaps buy one.

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 – 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).