I'm assuming you have 6 tracks on the timeline now, 3 video and 3 audio. Vegas uses a 'top down' approach to the video. Whatever's highest up is visible. When that goes away what's beneath it shows up. When you want to see what's on track 1 leave it just the way it is. If you don't want track 1 at some point, click on track 1 at that point and press S to split it. Place another split where you want track 1 to be visible again. Click the split out section in between and press Delete to remove it. Make sure you have ripple edits OFF or Vegas will pull the following part of track 1 back to close up the gap and you'll lose sync. Track 2 will now be visible during this section. You can add a fade out on track 1 at the end of it's section to crossfade over to track 2, and a fade in where it resumes. You can also drag transitions onto this fade area if desired. To make track 3 visible remove the sections from both tracks 1 & 2.

Audio ... i really think you don't want to switch the audio when you switch video. I'd suggest you pick the audio track that sounds the best and mute the other two. I'm guessing it may sound rather disturbing to have the audio change. However, if you decide it's necessary, do this with audio volume envelopes. On each audio track press the letter V to add the volume envelope which will appear as a thin blue line through the middle of entire track. Double-click on the lines to add control nodes. You can drag these nodes up and down independently of each other. Where you want to hear the audio from that track leave the level in the middle. When you want to mute it, drag the nodes all the way to the bottom to make it silent. Each volume change requires 2 nodes, one at the beginning of the fade and one at the end.

I do suspect though that overall you'll be happier simply picking one audio track and using it constantly through the entire video.