When was the house built?
If it waas built in the last few decades then it is standard to have an actual "subfloor".
Are you absolutely sure about this?
Do you have pictures of the floors?
How thick is the 3" wood?

well, the house was built in 1985, however, all the subfloors were particleboard. so, when i got the new floor installed, that's how they did it. maybe they did that so it wouldn't be higher than the adjoining 3/4"- particleboard-covered-by-carpet floor. now they're not a company anymore, so i have to figure out what to do about it. it's just normal, unfinished red oak flooring strips. I will post a picture. I painted it black. I want it painted black despite all the warnings that i should not. i'm re-doing the paint differently, which is why i want to know about fixing it. idk if you can see it in the pictures or not, but you can definitely see the paint wear where it is higher...

I see flooring laid directly on joists but it was generally done long ago and is the exception rather than the rule. If you want to keep it black or some other color, clean it, lightly sand it and apply a couple more coats.
I am a bit surprised how worn your three year old floor is. It must get a lot of traffic.

yes. lots of wear. dogs. children. not properly applied paint. but should i sand out the ridges or try to put some kind of vapor barrier underneath the joists under the house?

You really need to have a vapor barrier in your crawlspace. Even just plastic laid on the dirt without taping it to the concrete will cut down the humidity considerably. The humidity I believe is what makes the flooring cup. Most wood flooring is put over a sub floor with some type of underlayment that also cuts down on the vapor transmission to the flooring. I can't imagine this really working out in the long run.

Yeah, I have 3/4 inch oak floors over a 1 inch thick subfloor.

I recently put down a vapor barrier in the crawlspace 5 years ago after i moved in.

But, the floors do look pretty bad for only being 3 years old.

so. i have a vapor barrier directly on the dirt of the crawl space. my floors have had this ridgey cupping thing since the beginning. it does not change with the seasons or amount of rain or anything. should i staple a vapor barrier to the bottom of the joists and see if the ridges go down, or should i just sand down the ridges before i repaint?

I have a 1//2 inch subfloor and I used 1/4 inch birch plywood cut into 1 ft wide strips. There are 2 coats of water based, matte finish polyurethane on the finished floor.
I have small dogs with sharp nails and heavy adult traffic. My floors are still beautiful after 3 years.
The trick is..no shoes. All shoes are removed at the door. I could provide slippers for guest but most have socks without holes and don't mind the rule.
Shoes track in dirt and ruin the finish on flooring. You will get many more years of good wear from your flooring if you don't allow shoes.

I would not staple a barrier to the joists. Then moisture might get trapped. Remove the floors, put down a subfloor and lay some new flooring.

Do you own this house?