Shakespeare's "love triangle" sonnets: any other such poetry exist?

Hi all, apologies if this subject is in the wrong forum, badly phrased, or maybe too deep:

Many writers claim that Shakespeare's Sonnets involve a love triangle: I have been looking for an example (other than Shakespeare's) of sonnets which use a love triangle as its basis, without any luck: Perhaps Shakespeare's work unique in this respect:

Can anyone please help with this?

Best regards and thanks for taking the time:

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Shakespeare's "love triangle" sonnets: any other such poetry exist?