Merge Multiple Audio Files Into One Long MP3 File

No, not without undoing what you've already done, as they aren't in a clipboard, as such - you've just added some more file onto the end of one you've already got. If you want to have the ability to rearrange them easily, then you should use Multitrack view, and mix down your final arrangement. The amount of degradation (and there will be degradation if you start with MP3 files as well) will be the same either way, but you'll have more flexibility with the arrangements in Multitrack, as that's non-destructive. It also gives you much better options for the transitions between the files as well - cross-fades are easy, for instance - you can play about with them until you're happy with the result.

The reason for the degradation is that in order to open an MP3 in Audition, you have to decode it to wav - Audition's native format. And when you save the result as another MP3 file, you have to re-encode it. And that's a lossy process. The lower the bit rate of the original, the worse the degradation is.