uggh, i said emontx v3 shield. I should have said emontx arduino shield, those esp8266 modules look great but dont look applicable for the arduino shield
If there is a serial connection you should still be able to use the ESP. Just change the connections to suit. The 3phase firmware has the routine for transmitting over serial. You may need to patch your software as per the 3phase firmware.
If you go that route, you will need to observe the maximum serial data rate that’s specified in the 3-phase PLL sketch. I believe there are many such set-ups (emonTx + ESP) in Germany done by Simsala.