Exercising is good for the long term, but if the U/A is 3 days away, all exercising should be avoided.

The reason is exercising burns fat cells, releasing the stored THC metabolites back into the bloodstream. From there, the kidneys filter the THC metabolites from the bloodstream and deposit them in the bladder.

For the 3 days before the U/A, you want your body to revert to "storage mode" - stop all exercising and be as dormant as possible to avoid releasing a bunch of THC metabolites into the bloodstream, which will all end up in your urine to be detected.