Was suggested to us that we start with TinkerCad but Fusion 360 ( free edition) would be better, though a bit more of a learning curve; masses of ytube tutorials on both.
Again Cura seems the most popular slicer.
Octoprint with a Rasp Pi can be used as a remote controller and with a camera which we find very handy as our printer is in an outside workroom, but even used locally it allows you to upload your sketches without having to mess around with the little SD card as used on our Ender 3.
I've played with Sketchup quite a bit (just for designing - not building) but am looking for an alternative.
I've tried FreeCAD, and my head imploded. I loathe the hostage-ware philosophy of Fusion 360. TinkerCAD looks a bit primary-school-crayons, but I'm probably being unfair.
Am currently looking at Blender, but that's more movie-GFX than milling-machine-isometric, so I'm not sure that's the way to go yet.
I spoke to my SO and he says one can use just about any 3D program, he uses 3DS Max but says you could also use Blender or Maya etc. And for the slicing he uses the program that comes with the machine.