As a long time user of PD5 (PowerDesk) and for the last 3 years ExplorerPlus, I miss a few things when using Opus.
I have noticed that getting the Right Click on Menus or Icons rows to be very touchy in getting Customize to come up.
Most other programs are much more forgiving and usually allow me to click anywhere’s on the row and the Customize menu choices come right up. In Opus, you seem to have to be on a blank part of the row before the Customize menu is available which means I have to hunt around a little before having success.
So that left-clicking on the button copied a file, and right-clicking pasted the file.
So, because DOpus allows this functionality, right-clicking on a button is reserved for that, instead of displaying a context menu. However, as you discovered, you can click anywhere on an empty area to display the context menu.
You can also enter customize mode by holding Alt and clicking on a button, which will open the editor for that button directly and quickly. (This feature is optional so if it doesn't work you need to go into Preferences and enable it.)
You can also use the Settings -> Customize menu or create a hotkey to enter customize mode if you want. Alt-clicking stuff works best for me, though.
I would say about half of my buttons are 3 button buttons now. The more of them I set up the more uses I find for them. And the nice thing about it is if you forget what each 3 button button will do with either a left mouse click, a right mouse click, or a middle mouse click, just hovering the mouse pointer over the button will pop up a little tooltip telling you (providing you filled in the tip part of the button command dialog).
Another tip is to keep the button label of all 3 buttons that are within a 3 button button, about the same length. In other words have a general descriptive name that covers what it does and let the tooltips further define it.