M.L.
Mine is also 3 and my oldest is 5. The 3 year old does this, my 5 year old does a little more:
Clears the table after dinner (not all, just spoons/cups and such)
Puts silverware on the table
Holds dustpan while I sweep
Puts pillows back on the couch
puts dirty clothes in hamper
Sort laundry
Help me put clothes in the dryer
Put clothes in the drawer
Puts shoes in the shoe closet
Books books on shelf
Puts toys away (all day long, if it's not being played with, it gets put in it's organized storage so that toys aren't always everywhere)
Helps me pull weeds
Picks up pinecones in the yard
Dust
Wipe marks off of the wall or floor (like if he colored on a wall or spilled something)
Helps me cook (like get things out of the fridge, helps stir batter...)
Makes bed
Basically, he tags along as I'm working and I will hand him something small and ask him to do it, usually I have to help but he is getting better.
We don't do allowance, I used to give them a dollar about once a month, or every now and then I'll give them a coin if they've been super helpful. then, we'll go for ice cream. I don't have a set reward system, b/c then they learn to work it and I want them to help b/c it's nice, not just for rewards. But an unexpected reward is fun and nice for both of us.