I think the ADR system is wonky. I'm traveling solo but the majority of the restaurants didn't have any availability for 1. But if I plugged in 2 there was plenty of options! Maybe you need to try using 4 instead of 3? As far as I know, they don't have tables for 3 just like they don't make tables for 1. Like I said, I think the system is wonky.
Even when I called, the CM couldn't get me ADRs for places like Kona Cafe or Citricos. Places that I normally never have a hard time getting into. But the CM was able to offer me CRT, CP, and CM which are normally much harder to get. Go figure.