Argh it's tricky when they have the ability to escape but not the understanding why they shouldn't.
Do you think he would understand that in order to have a high back booster seat like his big brother, he needs to be sitting properly and stay in the straps properly in his car seat? Use it almost as a motivator like he's working towards that goal. That allows you to delay it for a bit (it's better if he's over 4, or at least much closer to 4 than 3) AND impress the safety behaviour on him before he goes into a seat which is even easier to get out of.
I wonder also whether you could say to him that he needs to use the harness seat properly OR use the shield type seat your friend lent. He can choose between those two options but if he can't behave properly with the harness then you will choose the shield for him.
Three is probably a bit too little to understand the idea of laws, but you might be able to talk about seatbelts/car seats being a rule for everyone that the police can enforce.
Some children this age respond well to positive reinforcement as well e.g. making him the "safety manager" of the car and making sure everyone stays in their seatbelt properly, and then lots of praise and attention for doing such a good job staying safe and making sure everyone else is safe too, or a reward chart for being safe in the car.
In terms of prevention, the prickly side of velcro stuck onto the button can make it less appealing to young children as it hurts their fingers when they try to open it. As a last resort, you can also cover the buckle, by loosely tying a scarf or similar around it, or the In Car Safety Centre sell a heavy duty velcro cover (designed for children/adults with SN) but understand that if you do this, it is introducing a potential risk in the very rare event you were in an accident where he needed to be removed from the car urgently by a stranger who is unfamiliar with car seats. The reason all car seats have a prominent red button which undoes all the straps at once is because in the early days of car seats, there was at least one case of a child unable to be rescued from a burning car because the rescuer couldn't work out how to undo the seat quickly. Also, beware that you are not adding bulk behind the straps as this can introduce slack which can be a hazard in an accident directly.