triple bagel

has a player already won a match 6/0 6/0 6/0?

On google I found:

Wimbledon 1987 - Stefan Edberg d. Stefan Eriksson 6-0 6-0 6-0

bruguera beat thierry champion in RG in the early 1990's by the same scoreline

Think the most recent one if you are including qualifying would be Woodbridge-Ortegren, Wimbledon Qualies R3 2001.

only time I ever saw anything close to a 3x bagel was Agassi winning 18 games in row against Hyung-Taik Lee 6-1, 6-0, 6-0 at the 2003 Australian Open 2nd round.

Novacek def Bengochea at 87 Roland Garros by that scoreline.

http://www.menstennisforums.com/showpost.php?p=1365335&postcount=32

also check the whole thread, you will find some more double bagels there added by other people:
http://www.menstennisforums.com/showthread.php?t=12484