Can I make porch light hot off an interior 3 wire switch

Depends on how the 3 way is wired. If power comes to the switch and there is a hot and neutral in the box, then yes. If the 3 way is a switch loop then no, there is no neutral. Open the box and use a tester to look for a constant 120 VAC hot AND a white neutral not connected to the switch. If you have that you can connect to those wires and makeup a 2 wire circuit.