from what i've heard the surf(4runner)/prado(colorado) and harrier(fortuner) have the same chassis, and that the harrier is a more "soccer mom" focussed SUV.
also harriers have 3.0 & 3.3L engines whereas the surf has a 3.4, 4.0 & 4.7L (petrol) and 3.0 (diesel) engines and the prado has all except the 4.7L too.
if you plan on going offroad then the harrier is the worst and on the limit (high speed) on road it has VERY soft suspension, both the surf and prado handle better (latteral G's).
however in the defence of the harrier, if all you want to do is drive around town and very VERY mild offroading, then the harrier is the cheaper and more comfortable option (value for money wise).
do note that the harrier is the smallest (interior cabin space) of the 3 as well.