McEwen is definitely the most suited to a middle role of the three.
Jermaine runs a decent line and has some skill but his real strength is his aggression and ability to dominate contact on both sides of the ball. IMO he was very effective in the first short stints he has done in the middle off the bench so far. Whether McEwen would be keen to make the switch to a 40-minute middle role when his best position is probably as an 80-minute edge is another question.
Lucas' current strengths are his line-running, defensive workrate/efficiency, and agility on the edge - In saying that, he definitely has the skillset and aerobic capacity to make an Isaah Yeo style transition to lock but it would be a strange decision to shift our current best edge who is signed long-term on big money to an entirely new position.
Manu would take years to become an effective middle and it takes away the freedom he gets on an edge to deploy his unique mix of athleticism and skill. I'm sure he could do it but it doesn't make sense for him.
I often try to temper my expectations when someone emerges as Manu has but I've seen enough, he's too good to not be playing 50-60 minutes on an edge next year. It's also not like he has emerged from nowhere, he was a dual-code prodigy rated as one of the best Union Schoolboys in the world, it appears he took some time becoming a professional athlete and is now reaching the potential that was always there - a natural talent ceiling higher than most playing the game.
Great problem to have for Holbrook/POS but I don't envy the responsibility of making these calls.
My "ideal" scenario would be that McEwen is keen on a shift to the middle so we start by running the rotation that Tom Gilbert used to do at the Cowboys when they had a glut of young backrowers coming through - McEwen plays on the right edge for the first 20 mins before joining the middle rotation with Manu playing out the game on the right edge.