I have DL some custom songs with “_rb3con” at the end. But the don’t show up on my system after I put them on my usb. Do I have to convert them in any way?

Did you follow this tutorial?

Check in System Settings – do you see the customs if you explore the USB stick contents under Rock Band 3?

If so, then you probably have TU5 installed, which blocks customs. Or you have the Mad Catz version of RB3 (either disc or on-demand), which also don’t allow customs.

If you don’t see them in the Xbox system menu, then you probably put the files in the wrong folder.

That makes no sense. “_rb3con” is just part of the filename. Remove it and nothing changes. That’s something some authors add to help you remember that extensionless file is a rock band song. But it does nothing.