What type of visa do I need should I want to apply for a conference in Canada. Please advise me for a 3-day conference.
For attending a conference you will need a business visitor visa. If you are from a visa-exempt country you will need to apply for an electronic travel authorization.
From another question:
If your visitor is from a country whose citizens need a visa to visit Canada, he or she must apply for a temporary resident visa just like any other temporary visitor to Canada. There is no separate application for business visitors. The temporary resident visa covers all visitors, including those in Canada on business.
You can read visa details and apply here.