Alright your question is all over the place. Let me try to figure this out.
You have a three phase panel. The different legs are all 120V when measured to ground. But if you measure between any leg to another leg you have 208V. In that panel you have no neutral wire. Just three hots and a ground? You cannot just say that the voltage was 120V on the bus bar without saying what you measured it to (to another leg or ground or what?)
Are you sure this is a three phase panel. The 208V measured from Hot to Hot indicates that it is but do you know how to tell? 3 phase is usually only found in commercial and industrial sites. Not in houses.
What is this box that the white wire goes into thats adjacent to the pole? Does the power go from the outside overhead lines, to a meter base (where power is measured and reported to power authority?) and than to your three phase panel?
Are you now trying to make your own 60A single phase sub panel from your three phase panel? I am still not sure if you understand how 3 phase works/is. We can help you but you need to be more clear.
I am not sure what PE wire is and I am not sure if you are talking about the main service conductors here when you are wondering. But XLPE has a clear almost like shrink tubing around it. If you were to scratch the outside of the individual cable it would sort of rip the outside. Kind of like it had scotch tape on the outside and you scratched it ripping some up. It should also say in writing on the cable XLPE if you can see any writing.
Balancing your panel just means putting the same amount of power on each phase if you can. So if you had 3 big assed air compressors, you would try to put one compressor on Phase A, one compressor on Phase B and one compressor on Phase C. (Or on Phase A+B, Phase B+C and Phase C+A if they are 208V). Do you see. So you just try to evenly space your big loads if you can. The smaller loads you don't need to worry about as much. But if you were doing a sub panel you would put the sub panel on a leg that doesn't have very many big loads on it in the main panel.
If you have two 60 breakers than on a short would trip the breaker that responds the best. Thats anybodies guess. Is your main panel a 60 A breaker and you would like your sub panel to be 60A? Or is the load your adding 60A and the main breaker is only 60A?
Yes you can use 2, 90 degree elbows together. Depending on your cable size it could be hard to pull through if you did them back to back. If you could put a short strait section between them it would be better. Would 2, 45 degree angles work in this application. Its not as harsh on the corners to pull through. Unless your cable is small compared to the pipe and than it shouldn't matter to much either way.
So you are missing tons of information. Let us know.