You're confusing "semester" which means "half of a (school) year" with "trimester" which means "a third of a (school) year (or even pregnancy, for example)". Those are two completely different ways of dividing a calendar/school year, while the word "term" is a general, well,
term that does not have a specific duration or partition size and thus can be used in both cases.
So as mentioned above, while a semester can be 6 months or ~4/whatever depending on what you mean, there cannot be 3 semesters in a single year - that would make them trimesters.