You should definitely take it in the summer. If you take it next year, you'll still be in Level II and will not be able to register in third-year Commerce electives (you'll only be able to register in required courses). You also will not be able to register in COMMERCE 3IN0 and as a result, will not be able to be in the Internship program.
COMMERCE 3FA3 is a required third-year course for which the prerequisite is COMMERCE 2FA3. This year, COMMERCE 2FA3 and COMMERCE 3FA3 were only offered in the Winter term, so you will likely not be able complete both courses in your third year. This will make registering for your fourth-year classes a lot harder.
More information is here:
http://ug.degroote.mcmaster. ca/reg...s-for-level-3/
I don't know how much this will help, but when studying for the exam, don't spend too much time on the textbook questions/additional problems in the coursepack as many of the questions are either ridicuously easy/hard or have incorrect solutions. Focus more on the the past midterms and final exams that are on Avenue. I've noticed that many of the questions have been either reused or slightly modified on the midterms.