Anti-Protoss Tactics - Get Airborne
If you can get air power early in a game against the Protoss, you'll be in a position of strength. Not only will you be able to stop the Protoss Zealot units, but you will be able to hold down resource-rich areas.
Anti-Zerg Tactics - Topple the Spires
Go after Spires. It's the thing that looks like a mushroom and lets the Zerg build air units. Unless your opponent is a believer in redundant buildings, destroying a Spire will set him back some. This structure is also required for building Spore Colonies. Take out a Spire and you can seriously cripple the Zerg empire.
Anti-Terran Tactics - Plague Them
Zergs can use the Plague to reduce the hit points of Terran infantry. By following up with an attack, a significant portion of Terran ground troops will be erased.
Anti-Terran Tactics - Uncloak
For Zergs, this means using the Queen's Ensnare skills to find cloaked units such as Ghosts or Wraiths.
Protoss can use an observer in their offensive troops as they scout Terran territory.
Anti-Terran Tactics - Spider Mines
If you even suspect that Terran Spider Mines are in your path, send in observing units to determine their exact location so that you can react accordingly. Those things are fast and deadly, and can wipe out your front line before you can say shrapnel.