I am writing a poem analysis of "We Real Cool" by Gwendolyn Brooks." I am having a hard time with my thesis- I have referenced the theme of identity and need to describe three aspect of elements of structure or poetic devices and tie it together.

If I understand your question, you need three aspects of the poem to tie to your understanding of the poem's theme of identity. If that is the case, then you might consider the following:
The speaker uses three verb tenses: the past when they assert "we left school," the present when they observe "we lurk late, strike straight, sing sin, thin gin, and jazz June," and the future when they predict "we die soon." In your analysis, you could focus on the actions as points on a continuum. Perhaps one action leads to the next?
Since the poem's speaker uses the collective pronoun "we," they are speaking for a group. Who encompasses this pronoun, and how do you know?
Analyze how Brooks designs her line breaks. Why do they begin and end the way they do? And why are all the words comprised of a single syllable?
When you answer the three guiding questions above, it will likely clarify your thinking about the theme of identity for the speaker and the others for whom they speak. The poem's final two words are haunting. Consider the tone. Is it accepting? Resigned? Defiant? How would any of those tones add to the speaker's identity?