SP3 contains everything from SP2. There is no need to, nor is it recommended to get rid of it. I am not even sure Windows would let you remove SP2 since SP3 essentially depends on it.
Yup, you gotta have SP2 in order to install SP3.
Well I gotta disagree STARS, Windows XP SP3 is incremental and not cumulative and needs at least SP1a to install and be fully updated, most service packs from Microsoft are incremental now as is the case with Vista.