3rd edition manuals for 3.5?

[Gwazi Magnum]

If we were playing a 3.5 D&D campaign would that mean that 3rd edition manuals are no longer valid for it?

Like for example Unearthed Arcana?

[dragonsamurai77]

Any 3e content is 3.5 legal unless it was updated for 3.5, in which case you're supposed to use the updated version.

[Curmudgeon]

If there's a 3.5 replacement or update, you use that (anything with the same name in the same category, such as feat or prestige class). There are also some official replacements. See, for instance, the list of spells replaced/updated/renamed at the beginning of Spell Compendium. And there's a list of official replacements from early in 3.5 here (http://www.wizards.com/default.asp?x=dnd/dnd/20050110x).

After that, you've got a couple of rules. This is from page 4 of the 3.5 Dungeon Master's Guide:
This is an upgrade of the d20 System, not a new edition of the game. This revision is compatible with all existing products, and those products can be used with the revision with only minor adjustments. Each individual DM gets to decide what "minor adjustments" they feel are necessary to integrate existing (3.0) material into their 3.5 game, so dragonsamurai77's "legal" statement is a bit misleading. What will work in one DM's game won't work in another's.

And, finally, the way you resolve disagreements among sources (including a mix of 3.0 and 3.5 books):
Errata Rule: Primary Sources

When you find a disagreement between two D&D® rules sources, unless an official errata file says otherwise, the primary source is correct. One example of a primary/secondary source is text taking precedence over a table entry. An individual spell description takes precedence when the short description in the beginning of the spells chapter disagrees.

Another example of primary vs. secondary sources involves book and topic precedence. The Player's Handbook, for example, gives all the rules for playing the game, for playing PC races, and for using base class descriptions. If you find something on one of those topics from the Dungeon Master's Guide or the Monster Manual that disagrees with the Player's Handbook, you should assume the Player's Handbook is the primary source. The Dungeon Master's Guide is the primary source for topics such as magic item descriptions, special material construction rules, and so on. The Monster Manual is the primary source for monster descriptions, templates, and supernatural, extraordinary, and spell-like abilities. As an example, when you come across Disciple of Dispater's Iron Power class ability saying it stacks with 3.0 Improved Critical, and the version of Improved Critical in the 3.5 Player's Handbook saying nothing stacks with it, the Player's Handbook is correct; Iron Power won't increase your critical threat range in a 3.5 game.

[Pancritic]

Using 3.0 materials in 3.5 can lead to instances of weirdness since certain things work slightly differently in the two systems, and can cause trouble when they interact in odd ways.

Personally, I forbid all 3.0 sourcebooks in my 3.5 games to save myself trouble, other DMs might allow everything in their games.

[Agent 451]

If we were playing a 3.5 D&D campaign would that mean that 3rd edition manuals are no longer valid for it?

Like for example Unearthed Arcana?

I'm pretty sure that Unearthed Arcana is 3.5. Unless you're talking about the other books of the same name.

[Curmudgeon]

I'm pretty sure that Unearthed Arcana is 3.5. Yes, D&D 3.5 came out in 2003 in the Summer months. Unearthed Arcana is from 2004 (February).

[Telonius]

A lot of the Unearthed Arcana stuff can be found at d20srd.org (http://www.d20srd.org/), under the Variant Rules section. (d20srd is one of the two go-to places to get 3.5 rules, along with the official Wizards site (http://www.wizards.com/default.asp?x=d20/article/srd35)).

[Fates]

It depends on the book, for me. While I allow stuff like oriental adventures and arms and equipment guide, because they're fairly simple and don't contain anything all too weird, I find that things like BoVD and BoED to be totally broken and not thought out well enough mechanically.

[hamishspence]

BoED is 3.5. Very early 3.5, but still 3.5.

A case can be made that few splatbooks are any more "broken" than the PHB is.