I didn't say he'd be defending threes.
Yep, figured you'd misread it.
TT or Lebron would defend the threes most of the time. And of course he could do that - who the hell do you think TT has been guarding in the Finals the last three years? He spent a lot more time guarding guys who basically had your typical three game (Barnes, Iggy, Draymond, and Durant) than he spent guarding true big men. And he even did a decent job sticking on guards for stretches with all those switches.
And KP was only in his 2nd season guarding NBA talent last year. He's been improving on coverage rapidly, and shown he can definitely guard threes like Giannis (while allowing that no one can really "guard" Giannis when he's on).
Watch here as he defends smaller players quite similarly to how Lebron does - repeatedly picking them up on the perimeter, where his size makes a shot intimidating, then blocking them from behind when they get by him: He strips smaller players trying to get by him surprisingly often too.
A lineup with Porzingis-Love-TT-Lebron-Shump would be able to do some scary things on the defensive end. It hides Love much better than the lineups they were playing him in most of this year.