As everyone said - the 3 names are Matthews, Marner and Hellebuyck.
Matthews - He's been called for years a top 6 player in the NHL (McDavid, Drai, Kuch, Mack and Makar). And rightfully so - I think he probably belongs in this group. The problem is - all 5 other players haven't only been good in playoffs, they've all played at an all-time-great level in playoffs to varying degrees. So - Matthews looks extremely lackluster in comparison.
Matthews doesn't just need to be "good" - he's previously been ok to good at times -but he now needs to be fantastic. Take over a playoff round - score ~7-10 goals, etc. Leafs have disappointed terribly in playoffs - and they once again have a team able to win the cup this year, but the only way they go on a run is if their best players step up and take over. And it starts with Auston Matthews.
Marner - same as Matthews. Expectations slightly lower for him, but flipside is he's probably been worst than Matthews in playoffs. He hasn't always only been bad - but he needs a consistently great run to help his team go far.
Hellebuyck. It took me a while to buy into the hype for him as a great goalie - but I've been really impressed this year. I expected him to crash and burn after last season, but instead he's doing even better. So - he's an all-time great goalie for regular season.....who has been shit in playoffs. He needs to steal a round, maybe two - and carry his team into a deep playoff run. I don't think it happens, but he certainly has a lot to prove.
Slight homer vote - I'll mention Montembeault for the habs. Not because he's disappointed in anyway like the other 3. It's more about - I still don't buy into him as a legitimate #1 goalie in the NHL, but he has been good still. Made the 4 nations. Won gold with Canada, etc. I think if he can be very solid in the playoffs (doesn't even have to steal a round, but just be good enough to allow forwards to possibly win games), I might be more likely to buy into him being a worthy #1 goalie.