For an even spreading, I advocate a barycentrical approch. It suffices to use the set of points:
with a double loop "for $u=0..20$, for v=$0..(20-u)$" (take care of this limit $20-u$).
Explanation: points such as the ones described in (1) are of the symbolic form $M=\alpha a+\beta b+\gamma c$ with $\alpha + \beta + \gamma = 1$ (check it...) ; they are inside the triangle defined by points $a,b,c$, and, in a reciprocal way, any point in this triangle is such that it can be written in this way ($\alpha, \beta, \gamma$ are called the "barycentrical coordinates" of $M$).
If you don't want the sides of the triangle to be "painted", use loops beginning at 1 and ending at $19.$
Of course, $20$ is just there as an example ; it can be replaced by any other integer constant.