Step 1: Recall why transmission systems avoid single phase.
Carrying large amounts of power over long distances with a single-phase, single-wire or single-phase, three-wire system would need very thick, expensive conductors to keep losses acceptable, and single-phase power also pulsates instead of being delivered smoothly.
Step 2: See why three-phase is preferred.
A three-phase system delivers constant, non-pulsating total power and can transmit far more power for the same conductor material, which is why every national grid, including the Indian grid, standardizes on three-phase transmission.
Step 3: Decide between 1-wire and 3-wire.
A three-phase system fundamentally needs three separate conductors, one per phase, so a valid three-phase transmission line must be a 3-wire arrangement, not a single wire, and it normally omits a neutral at the transmission level since the system is balanced.
\[ \boxed{\text{3-phase, 3 wire}} \]