How can I try BusyCal 3 without losing BusyCal 2

You can evaluate BusyCal 3 by having both BusyCal 3 and BusyCal 2 installed side by side as follows:

1. Open Finder and navigate to the Applications folder
2. Rename the installed copy of BusyCal.app (i.e. v2) to "BusyCal v2.app"
3. Download BusyCal 3 and run the installer.
4. Launch BusyCal 3 and go through the setup assistant.
5. If BusyCal does not automatically connect to your iCloud, Google or Exchange accounts, you may connect to them manually in BusyCal > Preferences > Accounts.

After you install and setup BusyCal 3, you can quit BusyCal 3 and launch BusyCal 2 from Finder > Applications > BusyCal v2.app (i.e. the app you renamed earlier). This way you can have both versions running side by side.