Use a command line/prompt for this one
Change into your Quake 3 Directory
type "cd demoq3"
then "edit q3config.cfg"
a MS-DOS like edit box should come up.
search for a line that says somthing similar to "seta model"....
Change this so the name before the "/" is "sarge" and when quake is run, the character you will be playing with is the sarge player.