To respond simply, your phone doesn't support 3 SIMs active at one time, so you wouldn't be able to achieve the functionality you're looking for.
On the Galaxy S20 series, you're only able to set 2 SIMs to be active at a time. You can either use: - 1 eSIM profile and 1 Physical SIM (and a microSD card) or - 2 Physical SIMS
You can only have 2 SIMs, of any form, active at a time. However, you can store multiple eSIM profiles and switch between them as needed. So maybe convert the SIM that you use least to an eSIM profile, and then you can turn if off when you don't need it.
Yeah, absolutely! You should be able to set up call diverting so any calls sent to your inactive SIM get forwarded onto your active SIMs. Just go into Phone>Settings>Supplementary Services>Call Forwarding and you should be able to configure the forwarding there 🙂