d1 = {key1: x1, key2: y1, key4: z1)
d2 = {key1: x2, key2: y2, key5: z2)
d3 = {key1: x3, key3: y3, key4: z3)

d_merged = { key1 : (x1, x2, x3), key2 : (y1, y2, None), key3 : (None, None, y3), key4 : (z1, None, z3), key5 : (None, z2, None) }