Visual c# Arrays and string

Do you mean that your file contains 7 lines of 3 numbers, or three lines of seven numbers?
Either way the principle is the same (I will assume your file contains N lines of three numbers, and you want three lines of N numbers as output). But, as this smells of homework, I won't give you code...
1) Set up three StringBuilders - one for each output, and string based separator, initially an empty string
2) Read a line from the file
3) Split the line (using string.Split) on the space character
4) Check you have exactly three parts, if not report an error and stop.
5) Add each part to the appropriate stringbuilder, using AppendFormat("{0}{1}", separator, splitpart)
6) Set the separator to a comma
7) While there are still lines in the file, repeat from 2.