Can 20 Credit hours be feasible?

Answered

If you have good study habits and time managements skills, 20 hours is feasible in most individual semesters, but it would be pretty difficult to do in every semester. Especially as an engineer, where you're not going to be able to mix your classes up and put together a blend of easier and harder courses like you might be able to in a more liberal arts-ish field. Not all classes are equal difficulty, and you're going to find some that take a lot more work than others; you'll inevitably have some semesters where those classes end up stacking. So if you find yourself in one of those semesters, taking 5-6 classes (what 20 hours usually amounts to) would probably be untenable.

You also may find, depending on the school, that 20 hours is over their credit limit. I checked on the options you mentioned and Memphis's limit is 20.5, so that's right at the edge of what you'd be allowed to do. Washington State however has a limit of 18 credit hours per semester—which doesn't mean you can't do 20, but you would have to petition for permission to take that many credits in any semester you were planning on doing so. What that means is that you need to have a reason compelling enough to do so, and you also need a track record of being able to manage that kind of load, so I doubt they'd let you go above 18 for at least your freshman year. Basically, certain schools may not even allow you to try this, and I would check on the ones you're applying to if it's your plan.

If you really want to graduate in three years, the best tool at your disposal is AP credit, because with enough of that you might actually be able to skip most of your freshman year (and then it's feasible). But outside a boatload of AP classes, 3 years is very difficult to do. And I'd honestly agree with the other answer on this thread—cost is really the only reason to try and graduate in 3 years. If you're going into debt or can't afford the school you've chosen for 4 years, then that's a fair reason. But if you're not putting yourself in a financial hole, you're only going to benefit from an additional year of college, because you are going to have more resources there—to build skills and expertise, to network and make connections that can heavily influence your future, and honestly to grow as a person—than you ever will at any other point in your life. I understand the urge to get in and out as quickly as possible so you can enter the world, but if you have the opportunity to go somewhere without taking out debt (which, if you're focused on public schools, ideally you should), take advantage of that for all its worth.

Aside from all that, regarding 5 year Masters degrees, those usually happen in fields where either the Masters program is only one year long (this is fairly common), or where, as a senior, undergrads can take a mix of undergrad and grad level courses. Occasionally there are 3+2-type programs but those accelerate an undergraduate major within a chosen framework and usually have a set schedule built so that students can manage them. So it's not quite the same thing.