basically
if i want to have a square whose bottom left point is 0,0 and top right is 1,1
the center of that square is .5,.5
but what if i want to have a triangle which is half that square exactly.
so its bottom left is still 0,0
its other points are 1,0 & 0,1
where exactly is that center of that triangle (it appears to PERHAPS be 1/3rd)
is it indeed
.333_,.333_
?
It depends on what kind of center you want, there are lots of different ones… 
One of the easiest ones (and probably the one you’re after) is the Centroid. You get it by taking the arithmetic mean of all points. So simply add the 3 points together and divide by 3. So if A, B, C are vectors of the 3 corners, you just do:
center = (A+B+C) / 3;
In your case (0,0) + (1,0) + (0,1) == (1,1)
(1,1) / 3 => (0.333,0.333)
However this works for any triangle.