The term 'Calligraphy Instrument' can be answered with the 3-letter word 'PEN'. A PEN is a writing instrument used to apply ink to a surface, usually paper, for writing or drawing. It typically consists of a metal nib that is held in a metal or plastic holder. Calligraphy is an art form that involves the use of a PEN to create beautiful, decorative lettering. Therefore, a PEN is an essential tool for calligraphers.