Winograd's proof of the lower bound for 2x2 matrix multiplication

There is the basic paper of S.Winograd (http://www.sciencedirect.com/science/article/pii/0024379571900097) about 2x2 matrix multiplication.

In the proof of the main Theorem 3.1, there is a some proposition that " ... each c$^{'}_i,_j$ can be computed using only three multiplications".

Because $\mathbf{c}' = R'\mathbf{p}$ and $R'$ contains 3 nonzero elements in each row.