Problem: You want your three year old to start taking a little bit more responsibility, but anytime you push a certain chore he rebels and refuses to do it.
Solution: This is a several-tiered solution. First, make it habit. Make it something he is used to seeing be done on a daily basis. We're attacking bed-making right now. When they see me make my bed, and on the days when I don't push them to make their own, when they see me making theirs, it solidifies the notion that bedmaking is simply something that is done. No question, no fight. The beds are never not made.
Second, make it fun. It's a quick task, so you can let them play in the covers first, be it making a fort or simply mussing them up. Then it's down to business, though. Make this routine so that they know the end result is always a made bed.
Third, make it easier. We put the girls' beds against either wall, since they share a room, but this makes bedmaking a bit too hard for them. If they have to climb over smooth covers to tuck in the sides, they're going to mess up those covers and have to do that part again. They're going to get discouraged. Leave about six inches of space between the bed and the wall, or whatever amount is small enough so that they won't fall in the crack when sleeping, but they won't have to jam the coverlet between the bedframe and the wall.
Otherwise, the "made" beds look like this: