Hello,
1. The price is a one-time payment which gives you a license key sent via e-mail. This license key is used to activate proper version (Master's/Home).
2. With the Home version, you can mix two tracks together to make 1 clip. You can then add background music the new audio file. If you would like to mix 3 tracks at once, you would need to choose the Master's Edition.
I hope this information is helpful. If you need more assistance, let us know!
-NCH Software