Click any of the 3 points and drag to move them around. The triangle should skew properly to fill in the area between the 3 points.

This takes the triangle created by any 3 points (which can be in any order) and breaks it into 2 right triangles, then rotates and scales them accordingly. It is possible to just actually flat-out skew a single triangle sprite in Flowlab (through a fun little bug, hehe) but it makes the triangle unable to be rotated afterwards, which is kind of a serious limitation.

Level 2: The bug I found that lets you just actually skew sprites in flowlab

Level 3 by JR01: 1&2 to change the skew angle, A&D to change the x size, W&S to change the y size, Spacebar to switch between skewing the x-axis or skewing the y-axis.