Pennant

[Sarah]

A small flag; a pennon. The narrow, / long, pennant
(called also whip or coach whip) is a long, narrow piece of bunting,
carried at the masthead of a government vessel in commission. The board
pennant is an oblong, nearly square flag, carried at the masthead of a
commodores vessel.

[Tariq Zaki]

A display poster with three visible sides.

[Rihanna]

a flag in the shape of a triangle

[Sommer]

A tapering flag on a ship, especially one flown at the masthead of a vessel in commission.