For courses of 3 months or less duration, you do not require a student visa and therefore the University will not issue you with a COE.
Applying for your student visa
You must apply for your visa through Department of Home Affairs (DHA). Depending on your citizenship and location, you may be eligible to apply online. Many students use the services of an education agent to process their visa application.
You cannot obtain a Student visa without a COE.
DHA must be satisfied that can meet certain requirements to study in Australia, and that you are a Genuine Temporary Entrant (GTE), before granting you a student visa. To be recognised as a GTE, academic study must be your primary purpose for coming to Australia.