This is purely my opinion on it:
If the question asks you to structure your argument from one angle in particular (eg "A study of Yeats' poems blahblahblah, discuss in relation to 2 additional texts") then you definately want to structure your argument around Yeats. Now whether this means talking about his stuff most, or talking about your other texts IN RELATION to issues and points brought up by your Yeats argument, is up to you (I'd do the latter).
I was taught to try and talk about my texts equally, but it can get a little fuzzy when you're integrating your argument, for example:
*Intro
*Point 1 looking at Text A (Yeats)
*Point 2 looking at Text A and similar issues in Texts B and C
*Point 3 looking at an issue brought up in Text A
as explored in Texts B and C
*Point 4 looking at an issue brought up in Text A
as explored in Text B in greater detail
*Point 5 looking at an issure brought up in Text A
as explored in equal detail in Texts A and C
*CONCLUSION
(NB this is a greatly truncated essay format, in real life it'd be a lot longer!)
It would appear from the above that you talk about Text A the most but in reality you would spend equal time on all texts - you simply narrow down your "essay points" to issues brought up by Text A.
Sometimes if the essay question is more generalised and asks you to refer to "____ plus two additional texts of your own choice" it's basically putting them all on the same level and whilst you should still try to talk about them in equal detail, you technically aren't required to focus on your prescribed text. In this instance I'd focus on whichever text is most appropriate for answering the essay question