hi Here is a quick suggestion (but rather long text). I think you may be using this already to some degree because you say you 'check him every 3 times in trot'

Try working in walk and trot and practising half halts first. If you haven't been taught half halts properly then now is the time to learn them. They work wonders. Often people cannot see that you are using them so it seems to them that you are using magic (a bit like horse whispering in fact)

If you use half halts in walk and trot and get the horse listening to them in those paces he will understand more about what you are asking of him when you use them in canter.

Here is a very basic explanation of what a half halt is for and how to do it.

You can use a half halt to tell your horse to listen to you. It can tell the horse that you are about to do something different. It can tell the horse to be aware that 'we are about to slow down' or 'speed up' or 'change direction' or 'ride a circle' - in fact anything at all really.

The aids for halt are used but only to a very slight degree and then are released again after about 3-4 seconds (this depends on standard of training etc)

Basically - the halt aids are
1. Sit tall - but not stiffly
Loosen your kneee and thigh if they are tight
2. Close your lower leg onto the horses side
3. Close your fingers into your hand to form a little fist - especially using your little and ring finger.
Do not allow your lower or upper arm to tighten as you do this.
That is all a halt aid is.

Now to do a half halt Hardly use any leg and
Hold the gently squash in the finger/hand to a fist while you count 3 or 4 and smooothly allow the hand to soften again.

That is all there is to it.

If he is busy in trot as well as canter the work on walk first. Do several of these whilst walking and changing direction. Do faster and slower walks - make him think a bit more and wonder what you are going to do next. Change direction a lot and do lots of circles and loops and different school movements. Look on the net for school exerecises. Use half halts especialy before a full halt. Reward each time by loosening the leg and hand when the horse does halt.

Then when you are sure he is listening in walk, do the same sort of thing in trot - lots of changes of direction - make him think of other things excpet going fast. Always initially use your voice at the same time - steeeeady steeeeeady each time you use the half halt aid. Do changes of pace from trot to walk etc. You might have to do several before slowing to a walk.

When he is listening to the half halts in walk and trot. YOu could try them in canter - keep relaxed and deep in the saddle as Sharon said (try letting your seat muscles spread over the saddle, that will make it deep) and soft reins as cherry chops said too.
Good luck and let us know how it goes. It is always good to know if it helps - or not !