In my personal opinion, if you hold a full time Diploma of 3 years, the best thing would be to apply for a full time Business related Bachelor's degree in a University or a college, but they usually require atleast, 60% throughout in your previous qualification (12th/diploma) and an IELTS score of minimum 6.5 (with no bands less than 6.0). Also most of the PG level certificate/ diploma courses in Canadian colleges require 6.5 IELTS score. You can also look forward to apply for 2years (fast track adv. dip) or 3 years advanced diploma in BM(or related field) with an IELTS score of 6.0.
Most PG cert/dip courses require a Bachelor's degree. Although some colleges/courses may also accept full time diploma holders. There are 2 colleges ( that I know of) accept IELTS score of 6.0 for PG cert courses, they are - Niagara college and Lambton college.
Go through this list of recognized participating Canadian educational institutions, and search all the college/ Univ. websites and select an appropriate course matching your requirements.
Given by information you have shared it's not easy to give you a relevant correct answer.
You have not specified whether the diploma is a undergraduate diploma or a post graduate diploma. Secondly, I agree with "Tiger" on the IELTS part of your question.