I'm not sure this solution is the best, but it's the only method I know within iTunes. It involves making a series of new audio files by transcoding sections of the original. So you first need to decide on a setting for iTunes > Preferences > Import Settings. I think the default is 256kbps AAC. But transcoding from lossy >> lossy always results in some loss of audio quality, so if this is an issue, maybe it's worth changing the setting to Apple Lossless, even if the original track is in a lossy format. Having done that:
1. Play the combined music file in iTunes.
2. Determine the desired start/stop times for each track, and jot these down. (For example, Track 2 - 3:22 / 5:47)
3. With the music file selected (highlighted), go to File > Get Info, or Control-click and choose Get Info from the popup.
4. Click the Options tab.
5. Enter the desired start/stop times for Track 1 in the corresponding fields and click Okay.
6. Control-click on the music file and choose "Create [chosen format] version" -- in a few seconds, you should see a new track with the same title as the original but a smaller number in the "Time" field. My habit is to rename these as I go, so as to avoid confusion later.
7. Go back to the original track, Get Info > Options, and enter the settings for Track 2 in the start/stop fields.
8. Repeat this for all the subsequent tracks. At the end, you can restore the original start/stop times to the original track (if you wish) by un-checking the boxes in the Options tab.