To put tracks 1,2 & 3 down to track four you need to turn the Record Function on track 4 to Bus Right. Set Record Func on 1,2 & 3 to SAFE and pan all of those right.
Put into Record and mix the three tracks. This mix is final and can't be undone if you later plan on recoding over 1-3 with new material, so do it until you like the mix. I used to add a little extra high end to those tracks as I bounce them to 4 because you lose some highs with the bounce.
To clarify the Direct function. Commonly people new to 4 tracking think that Channels = Tracks on a 4-track. This isn't the case. You can plug a mic into channel 1 a guitar in channel 2 and a drum machine into 3 and a room mic into 4, pan them all left and switch track 1 to buss left and all channels will be recorded onto track 1.
Let say this though. You want channel 1 (guitar) and channel three (drums) to be on separate tracks, now you have an issue with the Left Bus. If you pan both Channels left and turn track 1 and 3 on Bus Left record you will be recording a mix of guitar and drum on track 1 and the same mix on track 3. to solve this problem you switch track 1 & 3 to direct. This way Channel 1 ONLY will go to track 1 regardlessof pan and Only channel 3 will go to track 3 regardless of pan. Make sense?
I hope this helps. Sorry for the long post.
Chern