I've looked up several solutions for trying to play .at3 files and all of them have not worked. I have the codec installed and GoldWave won't open files with it. Every other solution requires using a PSP. I'm using Windows 7 64-bit. Is there any solution?

yes, Atrac3+ files can be played in vgmstream for foobar2000 without any other codec needed, and as far as I know, vgmstream will support normal Atrac3 files very soon.

if you can't wait until the next update, then you can get Sony Sound Forge and convert from there. of course, that's not the most practical way, but it works.

I tried foobar with vgmstream and it still couldn't play any of the Atrac3 files (they weren't Atrac3+ but just normal .at3s)

Like the man said, Sound Forge. If that doesn't do it, it may be you have an .OMG type file which is an encrypted form of ATRAC that has been renamed to a .AT3 file. *That's pure speculation though. :)

I tried foobar with vgmstream and it still couldn't play any of the Atrac3 files (they weren't Atrac3+ but just normal .at3s)
I just wrote that vgmstream will support normal Atrac3 files soon, which means it currently does not. Your best bet is to either wait until vgmstream is updated (should be soon), or use Sound Forge and convert to wav from there. I can help you with that, if you only need a few files converted.

BTW, did you download the at3 files from joshw.info's archive or did you rip them yourself?