TLDR: I think I have three opportunities which will materialize but the ones I want least are going to materialize sooner. How do I politely and professionally say "Hey I'm in the middle of a few interviews right now, can I let you know?"

So the situation I find myself in: I am out of work and have been interviewing/talking/networking a ton. The way I see it right now there are three opportunities that are in front of me that I think I have a shot at and I expect to have a shot at all of them. Assuming I could take my pick, I'm ranking them below in order of preference, starting with the opportunity I think I would prefer most.

• Opportunity A: Big, well-known company. Like, everyone's heard of it. Not FAANG, but close to that level. The role would be heading up a team in a new division. This opportunity is probably the most prestigious and will have the highest salary and greatest potential. The conversations here are early, but I was referred to the head of the department by a former colleague and so I think I have an inside track. I chatted with the department head who wanted to reconnect in a couple weeks for a few reasons (don't want to reveal too much here, but the reasons are sensible ones). So this may take a month or more to materialize if it does.
• Opportunity B: Smaller company, a startup, but one with a lot of momentum right now, great growth potential, and really strong financials. Obviously startups carry risk. This is probably the opportunity with a medium salary but likely coming with equity. This opportunity will probably take a couple weeks to materialize or get an offer for, but I would think it'll be quicker than Opportunity A. I know the CEO here and think they will do well.
• Opportunity C: Also a small company, a startup, but one that's a little further along than Opportunity B. This one is probably coming with the lowest salary (but could be equal-ish to Opportunity B) as well as equity. I'm meeting with the CEO on Monday for an hour and I expect this opportunity will be the first one to make me an offer (assuming an offer is forthcoming).

So, that's the background. The situation I'm in basically is that I do think I have a shot with all of them but they're going to come to me in the order of least preferred to most preferred. Obviously if I could wave a magic wand I think I'd take Opportunity A but if I wait and pass on B and C and A doesn't materialize, then I've got nothing.

So the basic question is: How do I kinda hold off B and C while waiting for A to materialize? What is a reasonable amount of time to expect them to wait. Basically I want a professional, polite way to say:

"I like this opportunity and I would take it right now except there's something else I have in the pipeline and I want to wait and see how that shakes out first before I commit to anything."