My DD went into a MaxiRider at 2 due to her being taller than a convertible car seat. They have an inbuilt harness, as to quite a few of the brands of booster seat on the market these days.
They changed the rules recently to go on age or height only as opposed to the old weight rules. They used to have to be in a 5 point harness until 18-20kgs. From the viewpoint of someone who has seen many many accidents involving car seats I will continue to have my children in a 5point harness until that weight.
3 is far too little for a regular seatbelt. Go to a fitting station or baby store and investigate a harness. I'm sure they'll also recommend a tethered car seat (one that is anchored) as opposed to one that just sits on the seat and secured by the belt the child is wearing.