The bottom bar, usually advertised as a tri bar or multi-purpose bar, works for anything you’d do with a neutral grip - hammer curls, french presses/extensions, overhead presses/floor presses/close-grip bench (as long as it’s not too narrow for your build. I’ve tried using them for “slow” kettlbell-type moves (presses, windmills, etc. Not cleans or snatches), it’s awkward, but tolerable.
I actually have a pair of them that I use as farmer’s walk handles. I wrapped a bit of athletic tape around the center of one long side (between the two “regular” handles) since it’s smooth, but that’s what I use them for the most. Sometimes I’ll also use them for “dumbbell” deadlifts.
The middle bar is the standard issue EZ curl bar. Versatile for curls, extensions, etc.
The top bar, honestly, I don’t ever remember really using.
If you’re building a home gym, I’d say get the EZ curl bar first since it’s more basic.