ROAR, the major sanctioning body for RC racing, specifies that you can only use 2 cell lipo for all 1/10th scale racing. Most tracks loosely follow ROAR rules for vehicle weights, dimensions, batteries, motors, etc... If you are interested, you can look at the rules on www.roarracing.com
That being said, there is no reason to use 3 cell lipo in 1/10th scale applications. You can get plenty of speed out of 2; just check out the fast 4wd cars and you'll see some serious speed on 2 cells. Most racing speed controllers are only designed for 2 cell and I'm not sure if the slash speed control will handle 3 cells. Additionally, a 3 cell lipo will contain 50% more cells, wired in parallel, to equal the capacity of a 2 cell lipo. This adds weight and size; that may not matter much in Slash, but the added 'girth' will make a fitment challenge in most AE/Losi/Kyosho racing platforms.
If you are looking for a good battery for your slash, I would recommend a good hard case lipo from orion, Intellect, losi, or other well known manufacturers. Look for something in the 35C variety. Most all packs, 3200 included, will provide more than enough capacity to run well over 10 minutes. The lower capacity packs, 3200 and 3800, are lighter than packs of higher capacity and should be the hot setup in slash. Google RC Market and you'll find a good site to buy Intellect batteries.
I hope this helps.