I think the easiest way would be to use a "While" loop. These take conditions similar to "if then". A while loop continues to loop as long as the conditions are true. If they aren't true the first time they'll never get executed (same as if then). If a while loop reaches the end of itself, it'll check the conditions again, and if they are still true, it will play through again (different than if then).
So you can use if boss health is > 75%. You'll probably also want to add boss is alive to each phase as well to be on the safe side. And you'll want some kind of wait action in your while loop (probably at the end) otherwise you can end up with an infinite loop that could crash the game.
Also if you want these phases in separate triggers, the following actions could be of use: "turn trigger on/off", "stop trigger", "run trigger".