You can apply after 3 years as a PR if you never left Canada during that time. If you did go somewhere on vacation, you have to make it up. You have 4 years to fullfill the 3 year residency requirement (1095 days). For example, if you stay in Canada only 6 months a year, you would never earn the right to apply.
You can even apply as soon as after 2 years as a PR if you were already in Canada on some other visa for the 2 years immediately before you got your PR. Each day in Canada before becoming PR would count as a half day towards the residency requirement so the 2 years previous to becoming PR if you spent them in Canada would count as 1 year towards the 3 year residency requirements.