Well, there IS a Nortel forum, but your question is actually generic. Have you tested the lines at the Minimum Point Of Entry with everything disconnected from the telephone company's terminal?? If the "heavy ground" is still happening, it is certainly Verizon's responsibility to correct. It it is clear until you connect the wiring back up, how is it when it reaches the Nortel system? Perhaps a problem with the wiring from the telephone company's terminal to the phone system? Finally, what happens if you swap Line 3 (with the ground problem) with one or the other lines? Does the ground problem stay on the Line 3 position of the phone system, or does it move with the phone number that was line 3?
Bottom line, grounding the phone system won't change anything, you need to discover the source of the fault on Line 3.