Tiering is based on what tech is available by upgrading your town hall in Warcraft... Zerg follows this, so tiering their units are relatively easy.
Zerg
1 (Hatchery tech, one building): Zergling
1.5 (Hatchery tech, two buildings): Roach, Baneling
2 (Lair tech): Hydralisk, Mutalisk, Corruptor, Infestor, Overseer
3 (Hive tech): Ultralisk, Brood Lord
And then there are a bunch of ways to sort the other two races, but generally the best way is by timing.
Terran
1: Marine
1.5: Marauder, Reaper
2: Ghost, Hellion, Siege Tank, Viking, Medivac
2.5: Thor, Banshee, Raven
3: Battlecruiser
Protoss
1: Zealot
1.5: Stalker, Sentry
2: Warp Prism, Observer, Immortal, Phoenix, Void Ray
2.5: High Templar
3: Colossus, Carrier, Dark Templar
Mothership: Mothership