I had two sets of batteries all last year. Actually, I only flew electric from June on... and I started with, and ended the season with just two sets of batteries... the same sets of batteries.

It's not a bad idea to have three sets, but it's not necessary. If you lose a set, or a cell within a set rendering it useless until it's repaired or replaced, then you can still practice with your other two. You only need one set for a contest really... A friend of mine did it for one contest, and was always ready for the next round without difficulty. The only problem he had was that his practice time was cut short... waiting for the battery to charge before each practice flights.

I have heard all the stories about people burning up a pack or two at each contest... and they are just that, stories. I along with a few others here in california did just just fine with just two sets of batteries.

But, three is preferable, though again it's not mandatory.