RU23 - Congrats on the offers! I'm a few years ahead of you, so not too far removed from remembering the huge relief that those summer offers represent.
Obviously, each firm has different practice areas in which it excels. I assume, based on this group of firms, that you are leaning towards corporate/transactional work. In NYC biglaw, the corporate departments get quite granular, and you'll generally work for a smaller practice area group within the corporate department (i.e. M&A, PE, Finance/Banking, Capital Markets, Asset Management, etc.). Additionally, you'll find once you're working that it's the group's reputation (more so than the firm as a whole) that affects your ability to develop business or move laterally or to an in house role.
Odds are, as a 2L, you don't really know which practice area to pursue within the corporate department, so you don't need to choose based on a particular practice area (unless I'm wrong here, in which case, do your research). Each of the Willkie, Cahill, and McDermott exist pretty much in the same reputational tier, in terms of transactional work (aside from bellwether practice groups) and they all pay lockstep.
This is what I know about those firms (and I'm just some guy):
Willkie is super fratty. High quality M&A work (probably better than the other two).
McDermott is a Chicago firm - consider what being at their non-HQ means. Beware of the blackbox bonus policy (while a few top billers may make above market; others may make below market - the hours in corporate biglaw are miserable, but there will be a gunners who live to work and claim those above-market bonuses. If that's you, great.) Good healthcare industry focus.
Cahill is solely a NY firm. Elite banking and capital markets, particularly bank-side. Lean staffing and demanding bulge bracket clients means rough(er) hours, but high prestige and exit opps in those groups.
It's very hard to determine over the course of a few zoom interviews, but ultimately go with where you're most comfortable with key people (think partners - there's a lot of turnover in the associate ranks) and the culture. They're all pretty similar though, and barring a disaster, you'll receive an offer from all of them.
It's a bummer COVID is going on, because the best part of summering is eating at a ton of steak houses all summer. Good luck!