There are typically three challenges that students have with online courses: dedication to being online and working daily for the course; the lack of direct interaction with the teacher; and the difficulty of the course just due to the material.
I normally recommend that students who have a 65% or higher from MPM2D (Grade 10 academic) should be fine in MCR 3U, due to the material, provided that they work hard every day in the course and finish recommended homework, complete assignments and study for tests.
However, when considering an online course, that mark should probably be 70% or higher, since you will still have the other two challenges. To ensure that you maximize your chances of success, you need to work through the instructor's daily exercises and homework, and be sure to ask your instructor lots of questions. Finally, I suggest that when students are taking an online course, then if possible, work with another student in the course to talk over any difficulties that you may have.
There should be the same level of assignments and difficulty in the online version of MCR 3U as if you were taking it during normal day school or night school.
If you can maintain the discipline of staying with the material and not falling behind, then the course may be challenging, but you still can be successful in MCR 3U. Good luck!