I would say play play campaign if you are into the story. Actually, play it co-op because it is a lot more fun. Campaign AI cheats and will not prepare you for multiplayer.
Skirmish is good to learn the build orders you want to use and what everything does. Although, AI cheats in Skirmish as well. For example, attacking resources in a multiplayer game is one of the most effective strategies. If you attack resources against an AI, you will not have much success.
I would suggest just playing Unranked matches against players. You can always hit me up and I'll show you some things. Here are a few tips:
- Don't try to build up forces and zerg. It is more effective to constantly harass your opponent in RA3.
- Destroying or slowing down their resources is a very effective strategy, especially early game.
I would also start with Soviets because they are the most straight forward.
