Some friends were talking about Red Alert recently, and I felt like it was worth mentioning here.
Red Alert was released in 1996, and was one of my favorite real time strategy games, rivaling the original StarCraft. There are only 2 factions instead of 3, but they both play extremely differently and are a blast to play.
The gameplay is simple enough: build up your base, build up your forces, obliterate the other guy. The tech trees and resource gathering are both straightforward, and you have access to classic units and structures, like the glorious Soviet Tesla Coil for base defense. Watching that thing zap enemy soldiers into piles of dust never gets old.
You can find this available as a free download nowadays if you google around, and there are even browser-based implementations of it, as far as I know.
Lastly, it has one of the best opening cinematics – and subsequently theme songs – ever. Hell March is the name of the tune, and it’s insanely badass.