The term 'Before Upsilon Letter' refers to the Greek alphabet, which is composed of 24 letters. The three crossword solutions for this term are TAU (3 letters), Phi (3 letters), and CHI (3 letters). TAU is the 19th letter of the Greek alphabet and is represented by the symbol τ. Phi is the 20th letter of the Greek alphabet and is represented by the symbol φ. CHI is the 21st letter of the Greek alphabet and is represented by the symbol χ. All three of these letters come before the 22nd letter of the Greek alphabet, Upsilon, which is represented by the symbol υ.