Hi,
Do you mean regions (the content on a track) or 3 x tracks worth of regions?
The easiest way to do the former is to select all the regions on the track and choose Edit > Join Regions
To do the later, I'd assume you want to mix down the 3 tracks into a single audio file? If so solo the 3 tracks, drag a cycle area from start to beginning you want to capture and choose File > Bounce > Project or section
cheers,
R
