My son has 3 subject tests - 800 Math 2, 800 Physics, 750 Chemistry.
Some of the selective schools he is applying to require 2 subject tests. Should he send 2 highest (minimum required) or should he send all 3 (showing breadth)?
He is not sure if the 3rd test will help him or hurt him.
There are certain colleges that require the applicant to provide the scores from all the subject tests he has taken regardless of how many are actually needed to apply for admission: Cornell, Penn, Yale, Carnegie Mellon, Harvey Mudd, Georgetown and potentially Bernard and Pomona (instructions for those two are ambiguous). Thus, if applying to any of tthose, he should provide all the scores. For others that require two subject tests, he should likely submit all three also simply because they are all high scores (that includes the 750). Most of those use the two highest to determine admission but it won’t hurt sending all three and Harvard and Princeton, though they state they emphasize the highest two, will consider any sent.
Thank you, @drusba for those specific and helpful examples. We have looked at some schools’ websites and they are not always clear on their subject test policies.
It sounds like you are saying that it would be beneficial to send all three whether the school requires it or not. I’ll let my son know.
Send them all. They are all good scores and the schools would look for the best scores accordingly to their requirement. There is nothing you need to hide.
It turns out that my son is nervous about that 750 score. Apparently it’s about the 72nd percentile. Should he still send it?
Apparently, some applicants believe that everybody is sitting with an abundance of free time looking for ways to use it 
Admissions Officers will not be evaluating percentiles on SAT Subject Tests. Send them all. It’s fine. If the college says they will look at the top two, assume that is what they will do. At least if your son sends everything, he’s not spending time creating charts of what was sent to which university.
Percentile is not an issue for subject tests. Sometimes even an 800 in a subject test is only 80th percentile. Because it is high level colleges that require subject tests, it is usually your high level applicants who take them with the skewed percentiles as a result. Subject tests are looked at to see if you are proficient in a subject. A 750 and even lower easily demonstrates that and the difference between getting 750 and 800 is minuscule.
Yes send all three by all means, a 750 chem is still an excellent score and it will not hurt only help show breadth