We had a triangle sneak in here. I'm not sure if that was supposed to say "square", so I'll answer it as such.
Let X = side of square
Then the sides of the rectangle are (X-3) and (X+4) and the area is (X-3)(X+4). This in turn equals the area o f the square, X2. Since the two are equal, we can set it to
X2 = (X-3)(X+4) and distribute to get
X2 = X2 +4X - 3X - 12. Cleaning up, we get
X2 = X2 + X - 12. Subtracting X2 from both sides and adding 12 to both sides, we get
12 = X
There is not enough information in the problem to solve it including the triangle. Asking for one side of the triangle leads one to assume that the triangle is equilateral, but that could be a false assumption. There is also no additional relationship between the square and the triangle or the rectangle and the triangle to be able to solve this problem for three shapes.