Fred Hicks (the dude who cofounded Evil Hat) talked about exactly this in an official blog post: One-Note Approaches in FAE. It's worth a read.
The bottom line is this: there is nothing that is inherently a problem about people primarily using their +3 approach. That's fine. If everyone's having fun, then everything is working well. The real problem, as Fred Hicks goes on to say, is if this makes things boring.
It sounds like things are becoming a bit boring and predictable for you though, so we'll go into how to address it. It's just important to first distinguish the real problem — if you find a way to have fun again, but everyone's still almost exclusively using +3 or +2 approaches, all is well here and there's nothing to fix.
Make the narration matter, and pick approach-based difficulties.
From your other question, it sounds like your group might not be narrating much and just saying "I shoot them forcefully" or something. This is a stumbling block: Fate is narrative first, which means you need to know what's going on in the game world, and the mechanics are only engaged to resolve what's going on. Gomad has written an elegant answer on putting fiction first that shows how this needs to work.
So, first step is: know what the narration is behind the action. It matters. What are they doing and what result are they trying to produce?1 Consider the story implications, whether it's practical or how hard it is, whether it may have unintended consequences, etc. Make those matter.
Then, once you're considering how they matter, set the difficulty based on what they're actually doing. That's covered in a sidebar on Fate Accelerated's GM section: approach-related target numbers.
That might look like this: If there's a large locked iron door, then breaking it down Forcefully is probably going to be of Superb (+5) difficulty or more. Further, crashing the door open (even if you succeed) might be really stupid and alert everyone to your presence. However, if they want to pick its lock Cleverly, that might only be Fair (+2) difficulty and allow for a quiet entrance. Suddenly using your Clever +2 approach doesn't seem so bad. If someone else has a Clever +3 and the time and ability to open the door, fine, good, they can do it. But in general, people won't always have the option of getting the other person to do it.
Often, certain approaches will simply be inappropriate for the task at hand. The Fate Accelerated chapter on approaches reminds you of this: you can't creep through shadows Forcefully, that's Sneaky. You can't push a big boulder Quickly, that's Forceful. Unlocking that heavy iron door Quickly is probably just out of the question — you need to use Sneaky or Clever or Careful.
Sure, someone can maybe come up with a contrived way to Cleverly jump across a chasm, but they may need to spend a fate point to suggest there's a rock overhead to hitch a rope to, or as a fellow player you can call that out as not seeming appropriate and suggest they need to use another approach. (You may want to suggest Quick or Forceful.)
1: This is a natural first step, then you consider what kind of mechanics and opposition mechanism should be involved if any.