Fichte is not absolutely necessary, but incredibly helpful as he functions as a direct bridge between Kant and Hegel. Fichte was the first to show that Kan'ts notion of "the thing in itself" was incoherent, and that therefore a step towards idealism was necessary. The tripartite idea of thesis, antithesis, and synthesis actually has its genesis in Fichte.
Schelling was Hegel's college roommate and was the one who convinced Berlin university to hire Hegel, who at the time was working as an undistinguished tutor. Much of the Phenomenology, especially the preface, is written in response to Schelling's ideas, most notably Schelling's idea that he could "jump" immediately to the absolute simply because he was a genius. Hegel's philosophy does away with the idea of immediate revelations, and replaces it with a long arduous process of dialectics. Infamously, Schelling ended his friendship with Hegel the second after the Phenomenology, and its damning preface, was published. After Hegel died, Schelling would go on to give a series of scathing lectures denouncing Hegel and his ideas. These lectures were attended by Kierkegaard and Marx, along with a few other important but more esoteric figures.
Still, from the perspective of pure philosophical ideas, Schelling is largely seen as nowhere near as important as Fichte, let alone Kant and Hegel. There has been a slight movement to take Schelling more seriously, especially from Zizek, but Schelling was infamous for constantly changing his mind, and his exact ideas are hard to precisely pin down.
In a *far* too simplistic form:
-Fichte was the one who came up with the self positing subject, which created the world around it through the necessary negativity caused by its own determination, note Spinoza's phrase "all determination is negation"
-Schelling was the one who pointed out that objects in the world were just as important as the subject which posited the world, and that the true absolute lies not in the subject itself but in subject object unity
-Hegel was the one to declare that Schelling's absolute was a "bad infinity", because it could not account for finitude. For Hegel, the absolute consists in both subject object unity, and subject object difference. He believes that in order to reach subject object unity while maintaining subject object difference, we need to go through an arduous series of dialectics, a path that has already been traversed by what Hegel calls the "world spirit". The phenomenology itself is basically a preface showing how we must interpret history to arrive at a moment in which we can then begin to do philosophy. But nobody takes the latter half of the Phenomenology seriously, as Hegel's interpretation of the consequences of his own ideas, namely that he believed 1800's Germany to be the "end of history", is quite ridiculous.