How to display a different relationship tab depending on some condition when opening a form

I'm working on the following:

Depending on one condition or another, I want to be able to show, say, the third relationship tab of a form rather than the tab with index 0 (or the lowest sort order) when opening a document form client side.

So far I've been able to recover the ID of the tab currently (and defaultly) shown, yet I haven't been able to display the third tab rather than the first. I haven't been able to find a method to display a tab with a certain index/ID/name/whatever. What am I overlooking?