Most heavy weapons, aren't one-shot, so once you land a shot on them, switch to a fast-firing weapon with a high DPS (automatic peacemaker, secret forces rifle, golden friend) to finish off their remaining health
Three cat spamming is basically quick switching different weapons and shooting them. Three cat spamming includes weapons from sniper, special, and backup catagories, and the guns that are used often have a quick bullet travel time.
Some ultra tryhards will five cat spam, or spam with all the different weapon catagories except for meelee.
The reason why people use 3 cat spam because if you are good enough, you can kill an opponent within 1 sec. I drop the game for a while because i reached that level where i can clean all people who get in my way and that is pretty boring.