Is solax Matebox compatiable with X3-ULTRA inverter?

No, it does not support X3-ULTRA inverter, because the Wire diameter is not the same for them. And also the water proof plug is not the same size. Matebox is mainly deisgned for X1-Hybrid-G4 and X3-Hybrid-G4 inverters. With using matebox advinced version, you can achieve whole home load backup.