Try concentrating on one triangle at a time. Perhaps you can imagine moving it around in your head so that it is a different way up?
You could print off this sheet of the triangles, then cut them out to sort them. (The first page is in colour, the second in black and white.)
Vincent and Tara are making triangles with the class construction set. They have a pile of strips of different lengths. How many different triangles can they make?