Just making a ‘toy’ like these triangles available to your young child builds creativity, to say nothing of the math concepts it actually teaches through the playing. That’s what I admire so much about the Montessori approach.
There’s a good suggestion to use the printable triangle templates with craft foam for durability, but even poster board will do.
This is an activity you will at least start by doing together- see the suggestions in the post. I’m betting your child, if old enough, will then take it further without your assistance.