Invasions are the hardest thing you can do in Dark Souls 3. It takes way more patience, strategy, and planning than anything else in the game to reach a point of reliably winning.
People who just blast into a 3v1 swinging and say it's unfair when they inevitably die are doing it entirely wrong. It's not a duel, it's a hunt. Stalk them, wait for your moment, attack and retreat. View it as a marathon and not a sprint. The best invasions are the ones where the host has thrown password phantoms and re-summons and blues at you for 10 minutes and has nothing but a panic attack to show for it.