I don't recall seeing US Airways flights on www.ual.com

There are flights from South America at that hour (e.g., United non-stop arrives about 6:30 a.m.; is that the flight you're on?). It's not just the # of flights but also # of agents on duty when you arrive.

You have separate tickets. Will your bags be tagged through to Florida when you check-in overseas? Will you get your boarding pass for all flights when you first check in? If not, and you need to go to the US Airways counters to check-in for your second flight, with your bags, you need to take that into account. If no bags to check, you may be able to check-in online. You will have to go through TSA security to get to your next flight, of course.

Also, if you somehow miss that second flight, will US Airways put you on another flight at no charge (of course, even then it would be the next flight with available seats) or would you need to pay a fee, maybe even a new ticket? I think 3 hours will be enough, as long as that first flight is on time but if you need to be sure you may want more time. What are your other options?