I don’t know how to describe this better, so I’ll just draw it:

(It took my 10 minutes to figure out how to draw this in inkscape)

What I’m trying to do is draw this curve, but ensure that curve is completely constrained by the triangle. When I try to use the 2-point arc tool, anything by the slightest curve extends outside the triangle.

(I’m using Sketchup Make 2017)