This is a two-sided worksheet for students to find missing lengths of triangles and then classify them as equilateral, isosceles, or scalene based on the lengths. I tried to find a worksheet myself, but all the ones I found also had classifying triangles by angles. I wanted to do a lesson first on sides only for my 3rd grade class, so I created my own. The students have to measure to the nearest 1/2 inch and I filled in the sides that were difficult to determine or closer to 1/4 inch, but you could easily white those out to make it harder or fill in the sides that are not whole numbers for a lower level activity. It covers 3rd grade CA State Standards MG 1.1 and 2.2. As an extension into angles, I will have them find the right angles, too.