Hey man,
So control in EDH is a bit different to other formats. Trying to control three opponents instead of one makes for a very different gameplay experience.
Because you are trying to control many opponents, simple counterspells lose some of their effectiveness. You can one for one with one opponent, but that leaves two unaffected. You can't possibly hope to counter every threat put down by three different players. As such, accurate threat assessment is incredibly important. Deciding what is actually worth countering is integral in any match up, but even more so in EDH when there are so many more potential threats. There is no real hard and fast rule here. You just have to do your best to identify your opponents strategies and win conditions and counter only things that allow those.
Furthermore, because of what I mentioned previously, a much more viable strategy for control in EDH is playing stax type effects. Things that increase the cost of your opponents spells, make creatures enter tapped, stop things from untapping etc. These effects are most commonly found on cards that effect all opponents, so you can get the necessary value from the cards. Your one card is effecting all three opponents at once.
Additionally, things like board wipes can do wonders in multiplayer formats. Allow your opponents to commit things to the board while not doing so yourself then wipe everything.
Ultimately, it's about tempo. You want to
disrupt your opponents plans enough that you can put your own into
fruition. That could be tax, tapping, bouncing, wiping. Many different strategies or a combo of many of them. And in EDH it is more so than ever about making sure your cards are doing their utmost to control multiple opponents at once.
In terms of win conditions, EDH doesn't suit the common hard to deal with late game beater that can be so effective in 1v1 20 life formats. As such, often alternate win conditions are a great way to close out a game. For example,
Approach of the Second Sun is an excellent win condition in Blue/White decks. Control the board until you can cast it, draw 7 and cast it again. Quite simple in those colours, even to do it all in one turn.
This has just been a brief overview of my own opinions. I hope it has helped some and I'm sure a lot of other users will have their own
insight to add.
Good luck on your pursuit for control.