There are two issues: 1) Your data has trailing commas, which are not supported and 2) The number of outputs parameter should be "3" as there are data columns, with the first column being the time column.
Original Data:
Time,U12,U23,U31,,,,,,,,,
0,0.05,-261.71,261.62,,,,,,,,,
0.001,5.37,-264.35,258.93,,,,,,,,,
Required Format:
Time,U12,U23,U31
0,0.05,-261.71,261.62
0.001,5.37,-264.35,258.93