Are 5-ht3 receptors found on inhibitory interneurones in the gut? and if so, then how do 5-ht3 antagonists inhibit colonic motility?

Good question: But most 5ht-3 receptors are central, the mechanism of action by antagonizing them is to control nausea, a central reflex. They do not decrease gut motility but receptor blockade does prevent nausea, a central reflex. Doesn't mean the receptors not in the gut, but 5ht-3 receptor blockers, zofran, (ondansetron) mirtazapine, do not cause constipation.