buy or upgrade? extra charge?

Absolutely not, every product from Skared Creations is "buy once, free updates", so we won't charge extra for any update neither minor nor major releases.

By the way the version 3 seems to be pretty stable now, we have some testers and didn't receive any issue feedback from long time. If you aren't already a tester for 3.0 beta and would like to take a look at it and try before upgrade your current projects then just fill the appliance form (you need to have purchased Combu on this website or have already redeemed the Unity Asset Store invoice).