Please help me with this one and explain to me how you found the answer
Gunther is looking like a stained glass window and notices that the design contains an equal number of triangles,circles,squares arranged in a step pyramid. The bottom first row has five pieces of only one shape. the second row has two pieces each of the other two shapes. The third row has three pieces of only one shape. The fourth row,has 2 pieces of only one shape. The top fifth row is a triangle. The bottom row does not contain a circle. what shape is in the fourth row?