I have a standing desk. I also have a photograph of myself slumped at it, taken by someone who thought it was funny.
The position is not the problem
Any position held for three hours is a bad one. Standing only changes which joints complain.
What helps
A timer set for twenty-five minutes, and a rule that when it rings you change something: sit, stand, kneel, walk to the window. The best chair is the one you get out of.