While tunneling around in the Nether I noticed that when I break Glowstone with a hammer it doesn't break it in a 3x3. I can only break one at a time. I am on version 1.16.5

It’s because hammer is like a pickaxe which only mines 3x3 if the pickaxe is the correct tool for the block. Glowstone doesn’t have a correct tool -> no 3x3