The ornamental design for a three-phase power tester, as shown and described.
FIG. 1 is a top, front right perspective view of a three-phase power tester showing my new design.
FIG. 2 is a bottom, rear left perspective view thereof.
FIG. 3 is a top plan view thereof.
FIG. 4 is a front elevation view thereof.
FIG. 5 is a right side elevation view thereof.
FIG. 6 is a left side elevation view thereof.
FIG. 7 is a rear elevation view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken line showing of structural features is included for the purpose of illustrating non-claimed portions and forms no part of the claimed design.