According to IK, SampleTank 4.09 and later works with OS 10.15. There are no plans to make ST3 compatible.

You might try the User forums to see if anyone has a workaround. I haven't found one but, admittedly, I've not spent much time there.
https://cgi.ikmultimedia.com/ikforum/

I have ST3 Max but hardly ever use it — haven't decided if the upgrade to 4 is worth it for me. I do see that Miroslav Philharmonik 2 became compatible four days ago.


BIAB 2023 Audiophile, 24/60 Core M2 Mac Studio Ultra/8TB/192GB Sonoma, M1 MBAir, 2012 MBP
Digital Performer 11, LogicPro
Finale27.4, Dorico5, Encore5, SmartScorePro64, Notion6, Overture5