How do you calculate the resultant 3-Phase voltage when each has a different reading close to 230?

There is no single 3-phase voltage that answers this question. Just as you have three different phase to neutral readings, you will have three different phase to phase readings.

Voltages used to described a power distribution system are 'nominal' values. For instance, the battery in my auto is a '12 volt' battery, yet fully charged it measures more like 13.6 volts, and that number changes with load and state of charge. We still call it a 12V battery. The nominal system voltage in your question would be the 230/400V 50Hz system used in Europe. We would measure about 230V phase to neutral, and about400V phase to phase. The individual phases could vary according to load conditions and transformer taps, but we would still use the nominal numbers when describing this system.

Since your question is in the singular, the answer would have to be the nominal system voltage of 400. The formula is:

single phase voltage * 1.73 = phase to phase voltage (nominal)

If you really meant the exact voltage on each possible phase pair, then easiest will be to measure it. If you really want to mathematically calculate the phase to phase voltages given three unbalanced phase to neutral values, that is a pretty advanced subject I will leave to the next answerer!