Jordan Walker, OF
Of the three players on this list, outfielder and super prospect Jordan Walker is the most likely to make the Opening Day roster. The Cardinals have an incentive to promote Walker early in the season so he receives a full season's worth of service and the Cardinals gain an extra draft pick if Walker finishes in the top three in Rookie of the Year voting.
My gut tells me the Cardinals would rather give Walker some seasoning in Memphis, where he hasn't played yet, rather than throw him in the outfield mix right away. If Walker plays in the major leagues, he needs to play on a near-daily basis; the Cardinals can't let him wither away on the bench.
If an outfielder scuffles or gets hurt, though, Walker is likely to be the first player up unless the Cardinals are really into service time manipulation, which the draft pick incentive is attempting to curtail. I do think that the Cardinals realize they're decent contenders for a title with this offense, and waiting on Walker for service time purposes would probably hinder their chances of competing for a title.
I expect Walker to demolish Triple-A pitching the way he's done at every other level. and when the Cardinals need an outfielder, he should be an instant lineup jolt.