Best to decide before first semester ends; To do a double major in 3 years, you'll pretty much have to use all of your free electives, unless some 1st/2nd year subjects overlap.
e.g. If you're doing a double major in Information Systems and Computer Science, usually most of the subjects in 1st and 2nd year are common, all you need to do is do 24cp of 3rd year subjects per major (i.e. no subject overlap).
alot of majors also share 1st year biology as common subjects, and all of them require 1st year maths.