Momentum of the other piece \( = \frac{M}{4} \times 4\)
Resultant momentum \( = \sqrt {\frac{{9{M^2}}}{{16}} + {M^2}} = \frac{{5M}}{4}\)
The third piece should also have the same momentum.
Let its velocity be \(v\), then \(\frac{{5M}}{4} = \frac{M}{2} \times v\)