Plotting is rather trivial. See the plot command reference.
There are shorter ways, but for easiest understanding just plot the three sides as three lines.
For instance if you have a triangle with the corner coordinates at (x,y) = [0,0],[5,10],[10,0]. Plot three lines between these points.
plot([0,5],[0,10])
plot([5,10],[10,0])
plot([0,10],[0,0])