A lot of that will depend on your school. At my school, RAs get pretty sweet perks, but they are also expected to do a lot. At the minimum, they must hold 2-3 programs per semester, have at least one informal gathering with 5+ residents per month, with more in the first month, keep an info board updated, create a new board about a topic of choice per month, sit on duty, do rounds, make door tags, enforce policy, write reports when policy is broken, help during move in and closing, check in and check out residents... There's more than that but it's enough to get the point across. However, this will vary A LOT between schools, and different institutions have very different expectations of student staff.
As for being a TA, that can also depend a lot on what class it is and what you are expected to do. As a sciences TA, you might just have to oversee a lab, give out quizzes written by your professor, and grade said quizzes and lab books. Or there could be a lot more to it than that. Or there could be a lot less. Also, this depends on whether or not you count this as part of your curriculum or not. Next semester, I'm going to be a research assistant, teaching assistant, and resident assistant, but the first two are three credits each and I'm only taking three other classes and a one credit activity course, so it's really like taking a normal-ish courseload.
Ultimately, you need to look into what the expectations really are specifically before you decide what you want to do, and also see if any of these things are specifically incompatible. For instance, you might not be able to be an EMT on Saturdays if you're required to do it every week, and you also have to be on duty on weekends sometimes as a RA (which you have to at my school). One thing I can say about student staff is that it might look like they don't do a lot, but there's a lot that goes on behind the scenes that you don't see, such as program planning (which can be complicated, especially if the program is large in scope), outside assignments, staff meetings, and ultimately having to deal with all sorts of problems that come up at unexpected times. RAs and other staff members do a lot more than you think they do.
Another important thing about being a RA is that often your time budgeting gets really goofed up. I'm not a RA yet, but I do have a different staff position and I work closely with them. Sometimes you might be studying for a test when you realize that there's a strong pot smell coming from one of the rooms on your floor, and then you end up having to call campus police and waiting around for an hour while they show up and arrest the resident for intent to sell (it has happened). Or you might be in the middle of an important paper and have a resident come to you in tears because she's fighting with her roommate.
That's not to try and discourage you from applying, because it is EXCELLENT work experience and teaches you a lot about dealing with people, good time management, and so on and so forth. I also know that my friends who are currently RAs (and also my boyfriend) have really enjoyed the opportunity to make an impact and describe the experience as super rewarding. Being on staff, we're very much like a family and I absolutely love the people I work with. But now I'm starting to get off topic...
Anyway, learn a bit about these opportunities, specifically by speaking to professors and people on the student staff to find out what the expectations of the different positions are. Once you learn more, you might decide it's completely reasonable, or you might decide that it's totally insane and decide not to do all three. For instance, it's extremely probable that your school's RAs don't have as much to do as they do at my school.
Good luck with making your decisions! You have plenty of time to think on what you want to do and I hope everything works out for you.