The bad news:
There really is no nightlife in Mammoth Lakes, unless you like drinking in small bars with a 10-1 guy to girl ratio.
its not half as bad as youre making it out to be, sounds more like you had one bad time
There are more than 3, and honestly for the size of the town and where it is geographically I don't think the night life is all that bad. Besides the ones already mentioned there's the Dubliner, Clocktower, Lakanuki, and a bunch of smaller restaurants that also have bars. Each place kind of has their own flavor, just depends on what you're looking for. And obviously weekends are the busiest, but there are some fun mid-week nights there too if you hit the right place.
Dubliner closed last season, that place was the shit. the night life in Mammoth is a lot of fun during peak season.... lots of hot tourists/foreigners. if bars arent your style its pretty easy to make friends and find house parties, theres always something going on