City Hall tour maybe, trip into Victoria Centre and up to the top of the dome for a look over Belfast, followed by a swift pint and a bite to eat in the Kitchen bar.
Or the Belfast Bus Tour followed by Kelly’s Cellar and a wander past City Hall and back up Arthur St.
Either of those should fill 3 hrs. Failing that bring the bike & head to Cave Hill.
edit: re Cynic-Al’s post. Stormont is on the bus tour but the building is not accessible, maybe by appointment. Titanic Centre is a modern tourist attraction but actually worth a visit. However 3 hrs on a Saturday might be cutting it fine unless you race round.