Just read a few responses and had to jump in with-
Three is much too young to consider using a booster (I'm not saying you were but it was mentioned in a response you got ...the minimum age recommended for booster useis really closer to 5 based on crash results (smaller children can suffer significant injuries from improper seatbelt positioning and boosters are really designed for much older children...even though the boxes say from 30 pounds)...
Anyway on to your question...we have the Apex 65 as a backup seat...our other seats are Marathons and I just don't think it's a great seat (and requires high head rests for use). It seems pretty flimsy...the armrest covers come off easily and there are holes in the armrest that my DD got her finger stuck in (they sent me tape to put over it- gee thanks!)
Like another person suggested...the Graco Nautilus would be a great choice...it harnesses to 65 pounds and the higher harness slots means it can probably be used until your son is about 5 and then it turns into a decent booster...for 150.00 a good investment and it's always good to have that extra seat.
Or if you wanted a seat that could be used by both your son and daughter try the Evenflo Triumph Advance- it harnesses to 50 pounds (she could use it rearfacing and son forward...the Nautilus is only forward facing so your daughter would be able to use it until much later-recent recommendations are to rearface well into 2nd or 3rd years).
Good luck in your search!