We have a pop-up menu with 3 tabs. All three tabs rank in importance. When clicked on an element in a page, you get a pop-up with the first (base) tab active where you see all the information you actually need. In some cases you want to click on another tab in that pop-up, although that's not that often. In even more extreme cases, you want to access a third tab, which contains very specific and somewhat sensitive information that we would like the users to access only as a last resort. In that sense, how can we make such tab less accessible to the user so he wouldn't enter it without the need for it?
This seems to be less making things "less accessible" and more like informing the user that this menu contains information which is not needed so often.
You could de-emphasise the tab visually, through colour and size, or even grouping by perhaps putting the tab to the far right and leaving a gap between the most common and least commonly accessed tabs.
Alternatively, you could look at your information architecture, you mention a hierarchy in terms of importance but your current approach displays each tab at the time hierarchical level. You could perhaps nest the data, so put the least important one as a subsection of the more important ones.
You also mentioned sensitive information, perhaps combined with the second approach you could use negative wording, colours etc to "warn" the user of this area is sensitive. This may cause the users to be wary of the contents. Sites like GitHub do this for destructive actions as an example: