You can certainly use 3.0e and 3.5e books together. There were many changes between 3.0e and 3.5e, mainly focusing around balance issues. Unfortunately no intentional balancing was done in 3.0, and as such one CR 11 monster would be easy, and another CR 11 monster might be a lethal encounter.
The biggest individual changes are.
- Ranger changes to make them playable.
- Druid changes to make them playable.
- Monsters gain skills and feats.
- Improvements to grid based combat, including the 1.5 diagonal rule (which was removed again in 4e).
- Changes to many spells. For example TimeStop can only be used to effect the caster. Before it could be used to destroy enemies entirely.
- No class-specific skills. Skills that are marked "exclusive" in 3.0 are "trained only" cross-class skills in 3.5.
The official change guides can be found here.
(Note: If anyone wants to add specifics on crunch to my answer, or copy-paste my answer to use as the basis for a better answer, please do so. I don't know enough to be helpful!)
I presume your approach is this: You want to try to combine the 3.0 core rulebooks with the 3.5 SRD, presumably because you want to use the 3.0 books for source material and advice but the SRD for stat blocks and an authoritative system of rules. You want to know what will you miss, meaning what could trip you up because you missed the difference between 3.0 and 3.5. I'm going to explain from a GM's perspective, that is, that you'll be planning a campaign, and that you've familiarized yourself with 3.5 already.
In short, as long as you remember to always treat the SRD as the only source for crunch, you won't miss much. A very small amount of mechanical data is missing from the 3.5 SRD (such as the stat blocks for the few monsters that Wizards kept out of the SRD, and the PHB's XP table). You can use the old versions safely, when necessary, provided you're mindful of systemic differences (such as the change to Damage Reduction). It'll be easy to know when to be careful, though, because you'll be looking at a book instead of a website.
Let me dig in to each core rulebook for specifics.
Player's Handbook. Honestly, just tell the players not to trust the 3.0 PHB and you'll be fine. Everything that isn't crunch is common cultural knowledge at this point, such as knowing that fantasy dwarves are good miners and like to use axes. The XP table is all that's missing, I believe, and it didn't change.
Dungeon Master's Guide. I actually own the 3.0 DMG but not the 3.5 DMG, even though I haven't GMed 3.0 in years. In fact, I reference it all the time for my Pathfinder game, for its general tools and advice on how to GM. So I think you'll do very well without the 3.5 DMG and your players will never notice a difference, provided you always use the SRD for actual game mechanics similar material such as the magic item chapter. But, say, the table of 100 plot ideas will never be out of date, you know?
Monster Manual. Effectively you'll be ignoring it wholesale except for, e.g., reading the colorful monster descriptions aloud to the players, and that's safe since they didn't redesign any monsters in terms of concept (that I know of). There is the matter of the non-SRD monsters, but again, you'll know to be careful when using them, so you'll be fine.
Best Answer
Mechanically, Pathfinder 2e is a completely different system. Even the things that share the same name are different enough that you can use basically nothing from 3.5e or Pathfinder beyond the ideas themselves—all the numbers, effects, conditions, costs, and so on are going to have to be redesigned from scratch for 2e.
Narratively, Pathfinder 2e has officially made Golarion the default setting, for better or worse, and of course it roughly captures a similar kind of magical fantasy world to those Pathfinder 1e was best suited to representing. So any narrative descriptions of setting content—particularly Golarion, but also really any other D&D setting—can be used with 2e about as well as they can with their original systems.