Voadam
Legend
I mixed and matched monsters and magic items and modules fairly easily among 3.0, 3.5, Pathfinder beta playtest, PF1, and d20M plus some other d20 systems (Arcana Unearthed, Everquest d20, some others) with minimal conversion but you had to decide which rule set you were using for rules adjudication because there were small pervasive changes throughout the rule system that varied between each such as the list of actions that provoked attacks of opportunity.Not for groups I played with. Organized play, yea but that was by fiat. Outside that? 3.0 and 3.5 content was used together a lot.
H*ck we used PF1 and 3.5 and 3.0 stuff too.