Linear algebra is dreadfully easy, though I think it can sometimes be overly complicated simply through the implementation of terminology that you aren’t use to. Just understand that it’s basic algebra but instead of numbers, you consider vectors. Vectors function slightly different as compared to regular numbers, but once you understand that difference, a first course in it is about as hard as a highschool algebra course is.
Calculus earns a reputation of difficulty amongst younger students because it usually follows a sequence of elementary algebra courses which basically teach you the same thing with greater complexity each sequential year. Calculus is the first new type of calculation that students are subject to for quite sometime, thus it’s percieved as difficult. Best way to learn it is to simply brute force your way through many problems until everything makes sense, but most students are lazy and unfocused at that level.
Ordinary differential equations will probably be the easiest course out of the three, but that’s under the assumption that you’ve already taken linear algebra and calculus, which is the typical course of action. It’s easy because a first course in it rests upon ideas that are ubiquitous in Lin algebra and calc. Just simple calculations really.
Partial differential equations, on the other hand, are quite menacing and work intensive, but I doubt anything but the simplest PDEs would be apart of a first course.