I'm a junior and taking AP Euro, APUSH, and AP Lang, and am considering taking APUSH and AP Lang exams, but not AP Euro. I'm also aiming for an Ivy League college.
You will receive normal credit for the class instead of being considered for the college credit. It wouldn't look bad, but you're missing out on those credits, meaning that's one more course you'd have to pay for in college! Something else to note is that Ivy Leagues will be looking for a rigorous curriculum, so the more AP, honors, and Dual Enrollment, the better. It is preferred for you to take that exam, but in general, they want to see you working harder when compared to others in your school. You also need to look at things outside of AP exams--volunteering, both in and out of school activities, your GPA/other test scores such as SAT and ACT. Good luck!
AP exams have very little weight in the admissions process - the courses are what matter more (as do your grades). It won't necessarily look bad if you don't take the exam, but you may miss out on college credit if you end up going to a non-Ivy (or equivalent school). You won't miss out on credit if you go to a top school, as most don't accept AP credits anyways, but keep in mind that some use exams as placement (though I imagine this is more the case for languages and STEM, not history).
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Thank you! I've decided to take all 3 exams.