Data in a file is best organised such that each row of data is separated by a carriage return. 2 or 3 numbers per line is recommended.
eg 4.555e-4 56.6 5 < cr >
23.4 34 0 < cr >
etc
If the y error number (3rd number) isn't known then don't put anything in.
eg 4.555e-4 56.6 < cr >
23.4 34 < cr >
Four numbers per line will not cause a crash but the first three numbers are put into the list and extra numbers are put into the next row and so on. (Thus you could put all the numbers into a single line).