I would keep Cates and Tippett over Frost right now. Though I'm happy to keep all three.
Cates has steadily improved in a tougher role, imagine moving from LW to center as a rookie and being assigned the toughest defensive matchup every night? And his offense has improved the longer he plays at center.
Tippett makes more things happen on a regular basis than Frost. He's a dynamic player who lacks the instincts to be a great player, but like TK can make things happen with his speed and is developing confidence in his shot.
Frost has improved with more PT, but it's obvious he's never going to be an elite offensive player. If he becomes more consistent he can grow into a 2C role - but he still makes too many mistakes and goes MIA for stretches. And he's a disappointment on the PP, he's just not a top playmaker on the perimeter like G.
Tippett has the most physical talent of the three, Cates is easily the smartest, Frost is between those two in both aspects. None is easily projected to a first line, but the three could be a solid 2nd line down the road.