3c

{\overline A CD + \overline E F + BCD = ( \overline A + B)\cdot CD + \overline E F = \overline { \overline{(\overline A +B)\cdot CD} \cdot \overline{\overline E F}} = \overline{(\overline{\overline A + B} + \overline C + \overline D) \cdot (E + \overline F)}}
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{ = \overline{(A \overline B + \overline C + \overline D)\cdot(E + \overline F)} = \overline{A \overline B E + \overline C E + \overline D E + A \overline B \overline F + \overline C \overline F + \overline D \overline F} }
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{ = \overline{A \overline B E} \cdot \overline{\overline C E} \cdot \overline{\overline D E} \cdot \overline{A \overline B \overline F} \cdot \overline{\overline C \overline F} \cdot \overline{\overline D \overline F} = (\overline A + B + \overline E)\cdot(C+ \overline E)\cdot(D + \overline E)\cdot(\overline A + B + F)\cdot(C+F)\cdot(D+F)}
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{ = \overline{\overline{\overline{A+A}+B+\overline{E+E}}+\overline{C + \overline{E+E}}+\overline{D + \overline{E+E}} + \overline{\overline{A+A}+B+F} + \overline{C+F} + \overline{D+F} }}