how do they write that on a computer? so the little 3 is up half a space?
Depends.
On an XP machine, go to Start->Programs->Accessories->System Tools->Character Map, and look for the "superscript 3" character. ALT+0179 also inserts this.
MC³
On this forum, you can use the "SUP" tags available in the formatting options on the Post Reply page, like this:
MC[ sup ]3[ /sup ] which produces
MC
3mystic_1