Firstly, those aren't the "big 3" marathons. At a minimum, London is by far the biggest and most important marathon.
On Mantz, he just has to keep showing up. Once you're in 2:04 territory, you have a chance. Since 2017 (the age of super shoes), 5 of the 9 winning times of the London marathon have been slower than 2:04. Over the same period, 5 of the 8 (cancelled in 2020) winning times in Chicago were slower than 2:04. He's fit enough to win but he'll need a little luck along the way. A 10% chance is probably right, but that's still.pretty high considering the depth of elite marathon running. There's probably only Sawe and Kiplimo that go into races with a far higher chance than that.