Am a PR holder since March 19 2016. I want to know if I can apply for citizenship?
Well assume you know that to apply for citizenship you need to show 3 years physical presence in the 5 years prior to your application date . You can claim up to one year as a temporary resident prior to being a PR at the rate of one half day for each full day.
If you only became a PR March 19 2016 with no pre PR credit then of course you cannot apply until after March 19 2019 assuming every single day you have been physically present in a Canada. Do not need the physical presence calculator to tell you that so you need to be a bit more specific with your question. The calculator itself is pretty self explanatory when compared with the challenge of applying for PR in the first instance.