Use a long length of coax with fittings to test the operation of the Actiontec units. Use it coiled up or run it down the hallways between the 2 send/receive units. Then you can determine if the problem is the house coax or the 2 adapters.
You can't really use CAT3 for useful Ethernet these days. 10BASE-T at most by specification. 10/100 if you're having a good day, but don't rely on it. Certainly not gigabit.
If you're not able to pull some new cable, and if wireless is insufficient, use MoCA Ethernet adapters, since you already have coax everywhere.