What? Parsec is a unite of distance not a speed unit, and you probably meant light years not light speed, and it's 3.3 because it's happened to be like that, it uses the astronomical unite aka distance from sun to earth a Parsec is the distance from the sun to the point to which you have an arcsecond (the point is on the axe of the earth orbit around the sun) I think you'll find better explanations on the internet than me trying to explain it to you.
Just know that Parsec is more practical to Astronomers than light year as it allows direct interpretation of data from objects you want to measure the distance to you in space
https://www.youtube.com/watch?v=uCSQd0eJKQQ
