What's the height for toddlers, for them to be out of a booster seat??

My son is over 20kg, and over 100cm (haven't weighed him in awhile). But he's only 2.8 years old. I was just wondering how big he has to be, to be able to sit in an adult restraint? If we have a third child, I'm wondering if a booster seat, baby seat and another baby seat will fit in a Holden sedan/wagon!!! Coz in my mind they don't fit!