All of the three listed programs are great for compositing. As far as picking one to start out in, pick the one you have access too and feel you will work best in. Similar results can be achieved in all of them, you will just get to that result in a slightly different manner for each one. One example being that fusion is node based and After Effects is layer based. If you use resolve frequently, you should already be familiar with a node based work flow. Many people that are skilled in photoshop that switch into compositing video choose After Effects as they are already familiar with the layer based workflow.
When starting out in VFX, I think it’s very crucial to focus on the concept of what you are doing. For example, if you are learning how to chroma key, learn WHAT you are doing and why it is happening. Don’t learn just how to pull a key in After Effects without understanding what is going on. If you understand the mechanics behind it, you will be able to open any keying software and have a solid idea of what to do even if you are unfamiliar with the layout or tools. So whenever you do decide to start in a specific program, learn what you are doing and try to understand it and you will best set yourself up for the future. Some programs handle different things better than others (like manipulating 3D data in Nuke versus animating elements for motion graphics in AE), but until you get to that point just learn the fundamentals in your software of choice. At the end of the day, the software you decide to go with won’t make your shot look any better. What will take your shot to the next level is understanding what you have to accomplish, and which tools you can use to get there.