According to the portal it is 5 days. I do understand that all flights into US are 3 days. https://usvitravelportal.com/
Hmmm, I do see that. Sorry for my "correction"! I could swear it was three days previously, but may be confusing it when we had to return test and were transiting through PR.
On our December visit, there was a return test required for transiting USVI. For February, we booked through San Juan to avoid that test, only to have the US add their own test requirement.