You need the angle bisector of all three points of the triangle, the intersection of those is called the incentre, and that will be the center of your inscribed circle.

Then, measure with a ruler the distance between the incentre and the side, and make interim marks that same distance away from the incentre, then you can trace it.

OTOH if your circle doesn't need to touch the sides, then just take a coin small enough and trace it in your triangle lol.