Gidon Ariel answered:

Thank you for your question! This passage is read publicly in traditional Jewish communities over a dozen times a year (twice on each of the six annual fast days) and therefore is often known by heart, but I bet many of my friends join me in admitting we’d never thought of that question;-)

I am happy to take this opportunity to plug one of my favorite resources on the Web, https://www.sefaria.org/Exodus.34.7?lang=bi&with=Commentary&lang2=en. That link brings you directly to over 150 commentaries on that verse! While many of the sources on that site are translated into English, sadly (for non-Hebrew speakers) many are not.  I quickly found the short commentary of the Bekhor Shor, who flourished in the second half of the 12th century. Quickly put, iniquity is when you were overcome by your evil inclination (yetzer hara); transgression, when you did a full blown revolt against God; and sin, when you did an action without intent.
Keep learning!