Brooks Lee probably has the highest upside of the three, but as already mentioned there are not Rule-5/40-man issues for him so he probably should be considered separately. If he absolutely kicks the door down, he'll be brought up. Otherwise let him develop at AAA.
I don't know where Severino plays. If Royce Lewis is considered good enough to be Plan B for everyday shortstop, were Correa to go down for an extended period, then there's room for Severino, given that Willi Castro has a minor league option remaining. If not, and if for instance we keep Farmer or intend Castro for that backup SS role, then it becomes a problem to have multiple 2B/3B players because it's harder to think of scenarios where they get enough use. Maybe Severino becomes a trade chip. It's nice though that Severino switch hits and doesn't seem to have a lefty-righty split to worry about. I like Julien and I think his defense has come along, but maybe he's the trade chip - except probably Polanco is the question mark, and I doubt he brings much in trade but might be the right one to move (sad to say). Complicated!
I have only the tiniest perception of Austin Martin in CF, from watching a grand total of one Arizona Fall League game where he wasn't really put to the test on any fly balls other than a can o' corn or two, nor aggressive baserunners testing his arm there. I know he is reputed to be fast, but he looked tentative and cautious out there, but like I said it wasn't much of a test, plus it was a long time ago now. (Better cautious, than a misguided hero mentality like my bias about Jake Cave though.) I'm anticipating he'd be adequate in CF, which isn't really a knock, but the bar is set high out there and we've been spoiled by Buxton and one year of MAT patrolling that area. Prepare to be disappointed if he's installed out there, at least until he gains more experience and becomes stellar at tracking the ball. I also note that they didn't really commit to him in CF at AAA, which maybe says something. If they see him as a 2Bman only, it just adds to the clog at that position, and a trade really needs to happen.
Of course being on the 40-man still allows for 3 years of minor league options. But taking up one of those spaces implies some degree of commitment by the team that wasn't necessary before, and constitutes a limit on what the team may work with, for the major league roster, in the coming season.
It's a lot easier to plug in prospects with uncertain defensive homes if you are coming off a 90+ loss season, since there'll usually be plenty of job openings for them to try to impress at.