I would wait and monitor the situation. The $100+ seats are not going to sell out overnight.

Start by selecting free 3 middle seats in the rear of the aircraft. In my experience, couples often select aisle/window hoping nobody will sit between them. If you fill the middle seat, they may switch, allowing you to take the seats. People may also upgrade, change, or cancel flights for whatever reason.

If enough time passes and the paid seats are starting to fill and no 3x has opened up, then you may take the advice of other posters and pay up (or gamble, your choice).