This is what is currently posted on the AA website

St. Thomas, U.S. Virgin Islands (STT)

Special notice
The minimum times for check-in and baggage check have changed for St. Thomas (STT):

For check-in: 180 minutes (3 hours) prior to departure

For boarding passes or to check bags: 90 minutes prior to departure

I interpret this to mean that if I check in online I only need to be there 90 minutes before departure to check bags.