Hello! I am working on a score (as a hobby composer) on a score for marching band. Normally I write four stk French Horn parts, but this time it feels better with only three parts. Is it normal that there is sometimes four parts and sometimes three parts for this instrument?

By "parts" you effectively mean players.

Usually pairs (2 players, four players), but occasionally three. (Beethoven's Eroica, Dvorak's Wind Serenade, etc.)