Past Perfect, as I was tought, is a structure used to emphasize the fact that one action had happened in the past before another action happened.
And this is pretty much what is happening in your sentence.
The order of actions were:
First - you hadn't wanted to buy this shirt
Second - you bought it anyway
That's why it is normal and natural for you to use the 3rd conditional form.
But then, why the mixed conditional has to be be used? here's why...
The 2nd-3rd conditional is more general. The first part of the sentence in mixed conditional has to be constant and that's what is happening in this sentence.
The person didn't want to buy this shirt and they probably still regret doing so and that's why the action (not wanting to buy the shirt) is expressed in the 2nd conditional.
Answering your question: when could you use a pure 3rd conditional?
Well, in the moment when it is emphasized that the person doesn't regret buying this shirt after all.
- Nice shirt you have here!
- Oh, thanks. At first I hated it, but now I love it! It's so cute.
- Well, If you hadn't wanted to buy that shirt you shouldn't have bought it
- I know, it was stupid, but I got used to loving it actually.