There are utilities which will allow you to concatenate mp3 files without decoding/re-encoding the files, so there's no loss in quality.
If you want to do more than than, i.e. actually cut out a segment from one and concatenate it with another file, I believe you'll need to decode the data first using some editor which understands mp3 (e.g. Audacity). This will degrade quality since you then need to re-encode it after doing the cut/paste work.