Get line to line values from line to neutral reading
If you can rely on the three phase voltages being 120 degrees apart then you can vectorially add two phase voltages to yield a line voltage: -



The above shows three phase voltages VR, VB and VY. these vectorially add to produce the line voltages shown.

Clearly, if the phase angles are not exactly 120 degrees apart then there is some error in this technique. You cannot tell what the phase angles are from the magnitudes of phase voltages so unless there is some phase data that can be used, I think this is all you can achieve.