Championships I have witnessed:
Yankees - 7 (1977, 1978, 1996, 1998, 1999, 2000, 2009)
Cowboys - 4 (1978, 1993, 1994, 1996)
Bruins - 1 (2011)
So 12 total.
I also rooted for the Celtics when I followed basketball in my teens, so thought about adding 1984 & 1986, but I dont think I've watched a complete basketball game in 25 years so I left them off.
An odd group of teams, to be sure. My dad has always been a huge Yankees fan, so he got me hooked early. Cowboys were the first NFL game I watched on tv. We used to get WSBK out of Boston on our cable system here in NY when I was a kid, so that got me watching the Bruins and Celts in the very early 80's.