Are there any pre-made campaigns that are fairly simple and able to be played by a group of three people for D&D 3.5? My friends and I have a hard time getting a campaign that we can play, as it gets difficult quickly when we only have 2-3 characters. My price range would be fairly cheap; from free to $10 at most. I would prefer a shorter campaign that would last a few 2-3 hour long sessions.
When I started playing d20/3e D&D, I ran my friends through The Crucible of Freya, and I remember it being a lot of fun. Frog God Games offers a version for 3.5e in PDF for $2.99 USD. Here's a good capsule review.
When Crucible was released, there was a free prequel adventure called "The Wizard's Amulet" that I ran first, and it really hooked my friends (that's why I bought Crucible of Freya). I couldn't find a conversion to 3.5e for sale anywhere, but a google search turned up many places where the original 3e version can be downloaded.
One caveat: there was a third adventure in the series called Tomb of Abysthor that really fell flat. On paper, it looks fantastic--especially if one of your players wants to RP a character turning into a paladin, but the actual adventure was a slog-and-a-half.
Keeping this away from "list of cool adventures", I think looking for specific small-group adventures might be the wrong tactic anyway. The right move would be to find a plot that the group will enjoy, and then go through the stat blocks and downscale as necessary.
Most adventures will give you the CR for the encounter - take that number, do the math to see what the actual CR would be for three characters, and then downsize accordingly to bring things back in line. (If they don't, you can get the number by just running the math for a 4-5 person party)
Money and treasure also needs to be downsized (mostly money). This is also a great time to tweak the particulars to suit your party.
Watch for encounters that assume skills you may not have. Adventures tend to assume you have the four basic Monster Food Groups (beatstick, arcane, divine, skill monkey). A three-person party will either be missing one area entirely (e.g. no rogue), or if multi-classing will be weak-ish in two of them. So you'll need to tweak the areas of the adventure that are looking for those skills or class abilities.
Following the above, consider some sort of "cheat" to cover the missing group. An NPC "hired help" for a fighter, that sort of thing. Ironically, while I would normally insist that a 3.5 party needs-needs-NEEDS a cleric, if you're willing to have a world where potions are cheap, that's often the easiest role to skip in a three-party.
I want to be careful about turning the answers to this question into a list (which is a generally upheld no-no).
That said, I like @cr0m's answer as it certainly hits all the criteria you lay out in the question. My answer is going focus on "throttling" published campaigns for smaller groups and provide some starting points looking at other options.
In my experience throttling is easily said, but more difficult in application, if only because many campaigns as published seek at least twice (2x) your group size, i.e., 4–6 players. As published also means that a "balance" is baked into the campaign along with level progression, magic item distribution, and wealth. Take any Piazo Adventure Path and play it with 2–3 players and assuming that the group makes it to the second installment without GM intervention, the characters will be rich—either from all the treasure or from selling all the excess—and possibly a menace...from a campaign that unexpectedly turned "Monty Haul."
My experience with Pathfinder GMing proves that much of the balance can be worked out in any published adventure mathematically in the CRs and ELs of the encounters directly and fairly to all involved.
Seed or Prime a Sandbox Campaign
Alternatively, try a Sandbox Campaign. I love sandbox campaigns as a GM and have started numerous campaigns just by outlining a major, possibly world changing, threat and seeding the first story arch with a smaller published adventure where the threat is introduced, possibly in person, and the characters choose to engage or chase this threat or threads of immediate satisfaction for character or background hook.
Sandbox Campaigns are more work for a nascent GM of course, but can be immensely satisfying for all involved. Moreover, as a GM the campaign is a growth engine for mastery of story telling and player engagement with characters. I highly recommend the Odyssey: The Complete Game Master's Guide to Campaign Management from Engine Publishing for veteran and new GMs of sandbox campaigns.
Lastly, Adventure Paths are outside of the price criteria—six (6) installments at $16US each—so you will possibly want to turn to the OGL/PDF market. I think two specific channels will be of direct help: 1E/2E/3E conversion and the home-brew campaign efforts by a myriad of GMs. In the former, for example, The Temple of Elemental Evil, Against the Giants, and D1-D3, Q1 are now available as cheap PDFs on DriveThruRPG with fan-conversions to 3.5E readily had, the latter is a mixed bag of good and awful, but there exist both free and paid options on DriveThruRPG.