bump max builds to 3 on bots

[brson]

Right now we are running 8-core/15GM VMs but only 2 builds at a time, usually with -j2. It seems like we could easily bump this to 3 builds at a time to improve our utilization.

[brson]

We might also switch back to high-mem instances, which have 30GB RAM, but slightly slower CPU's. That would allow us many more builds.

[brson]

I've bumped the builds per slave to 3.