If you're talking about FR materials, I guess it all depends on where you're setting the campaign. Faiths & Avatars and its sequels are great, and although the upcoming Faith & Pantheons book is aiming at a lot of the same territory, from what I gather it seems like it will have a lot more stats and a lot less information about other aspects of the religions, so F&A might still be worthwhile.
As for regional accessories, I'd say go for the first generation (FR1 - FR16 series) if you'd prefer more detail than the campaign set but still a lot of room to manuever. If you're looking for lots of details, go for the second generation releases (The North, Lands of Intrigue, and Empires of the Shining Sea and Spellbound boxed sets). My personal favorites are FR5 Savage Frontier and Lands of Intrigue.
If you're talking about core materials, you'd probably have more trouble using them in a 3e campaign since they're more rules heavy. Player's Option: Spells & Magic has some variant magic systems that looked interesting but I never used them so I can't say if they play well.
Also, one of the best products of the 2e era was the World Builder's Guidebook, but it might not be that necessary for you if you're planning on using a published setting like the Realms.
On the same note, some of the other campaign settings are worth looking at but you probably don't need them if you've already decided on the Realms. Personally I prefer Birthright and Dark Sun.