It all depends on what level of support you wish to provide for poorly defined customer's models.

I do professional CAD on the higher end personal CAD systems. These are not for the faint of heart. The learning curve on these can be steep with rewards equally as impressive.
If you want to try out CAD, get the 30 day loaners most companies have even if you have to jump through a few hoops. You'll know quickly if this is for you or not. If you have educator or student credentials, you can cut the cost significantly on the major brands.

Otherwise you are looking at two other pieces of code you'll need; a good slicer. I assume Ender 3 comes with decent enough support support (!ha) but if you find that you need more "access" you could plop down the token monies for Simplify3D. But facet manipulation software is something that has little direction or attention. Most CAD systems completely avoid STL files except for export or import reference only. STL, OBJ, WRL are all part of a standard started nearly 30 years ago. They are called facet files and much of their core is thanks to VRML development. From a layman's point of view, VRML is the core of why we can do 3D graphics in such lightweight GPU's. The tech is over 30 years old!

You can get MeshMixer free. A good 3D file manipulator that has capabilities of creating custom supports. MM has geometry manipulators out the wazoo as well as primitives for creation. Probably the best no money can buy. After that, you get into some really expensive (relative of course) scene generators for 2D rendering and such. Way more than anyone would ever need for a simple 3D printing parts manipulator.

Don't forget about the cleanup and the intricacies for what a FDM printer like the Ender 3 can actually accomplish. You're going to get that angel request with delicate wings that a father will simply have to have at any cost, perfect for his little daughter's birthday... tomorrow! Are you going to take the job? I get very choosing in what I will attempt on my printer and what I won't. You might find that there are more hands-off printers out there for a lot more money but end up being a whole lot cheaper in the long run.

One day someone will pull all these things together in a better platform but for now, Autodesk is probably the most in-tune with actual users. Fusion 360 is a good place to start with CAD. Then again, you could consider hiring a CAD person that has demonstrated 3D printing skills and offer them a dream job in 3D printing