Can u stand on 1 leg?

 
Does your horse stand on 3 legs easily?
If they cant - why not???

It's important to get under the horse and  pick up each foot and see if they:

1. struggle to square up over the other 3
2. can stand for a normal length of time without teetering or asking for their foot back
3. have a comfortable range of motion
4.  They USED to stand on 4 and suddenly cant

You should regularly assess this (owner/trimmer/horse ).

Why??  
To see that movement continues to be beneficial. This is a good way to stay on track with a successful transition from unhealthy feet to healthy feet and to keep moving forward with continual development.

And thats not the only reason...!!!!!

Also - a sore horse will show an unwillinginess to unload/load to another foot.  Your horse is an athlete - performing for you and doing stuff they may not normally do at liberty.

Reasons - could be hoof, leg, shoulder, the opposite side, the diagonal.
Dont forget inflammation - this also needs consideration.  Stress, Diet, lack of movement - the list for inflammation can be endless.

The take home message really  - is to never ignore the horse that is unable to stand 3 legged for ANY period of time.