So, I am currently a freshman at Westminster College and I am pursuing a major in Biology and Chemistry. I was debating if I should add Environmental Science (ES) as my third major. My current majors overlap quite a bit when it comes to classes, and if I were to add ES as a third, there are only 5 unique classes I would have to take to obtain the degree. Should I go for it, or should I stay as a double major?
The name of your major(s) matters much, much less than you probably think- there are no gold stars when you apply to grad school or jobs for having a triple major v a double major v a single major. What will matter is how you did (grad school and jobs) the actual courses you took (mostly grad schools), and your internships (grad school or jobs). So, it might be a fine idea, it might make no difference, and it might be a bad idea- depending on you, your interests (current and longer term) and your ability to carry a heavy course load.
Be aware that for many students, first semester first year is not a good indicator of how hard / easy college work is going to be. It is very common for students to sail through the 1st semester, and then find in 2nd semester first year or (more often) first semester second year that the weight of the work has ratcheted up a lot more than they expected.
Afaik you have until 2nd semester 2nd year to declare your major(s). Get through this year / talk to your advisor and see how things develop. If you have time on your hands right now, put it into lining up a good summer internship (REU applications open in December) to test out your longer term interests.
ps, one benefit of taking classes in a subject but not majoring in it is that you get to choose the classes that interest you, not all of the required ones!