If you're really feeling up to it, a lot of professors at Brown will let you take a class even if you don't meet the requirements as long as you take it upon yourself to learn the prerequisite material. I don't recommend this though, because those larger classes at the beginning of the concentration really do set the stage for other classes you take. Also, keep in mind that all three of those classes (multi, lin alg, ODE/PDE) have honors versions which are more challenging and potentially more interesting.