You just have to remember these:
a+b=!a*!b
a*b=!a+!b
plus of course !(a+a)=!a, !(a+0)=!a, !(a*a)=!a, !(a*1)=!a

So for example 3 input NOR = !(a+b+c)=!((a+b)+c)=!((!a*!b)+c)=!(!(!a*!b)*!c)
Hope this helps

You just have to remember these: $$\begin{align} a+b & = \overline{a}\cdot\overline{b} \\ a\cdot b & = \overline{a}+\overline{b} \\ \end{align}$$

Plus of course:

$$\begin{align} \overline{a+a} & = \overline{a} \\ \overline{a+0} & = \overline{a} \\ \overline{a\cdot a} & = \overline{a} \\ \overline{a\cdot 1} & = \overline{a} \\ \end{align}$$

So for example a 3-input NOR gate is:

$$\begin{align} \overline{a+b+c} & = \overline{\left(a+b\right)+c} \\ & = \overline{\left(\overline{a}\cdot\overline{b}\right)+c} \\ & = \overline{\overline{\left(\overline{a}\cdot\overline{b}\right)}\cdot\overline{c}} \end{align}$$

Hope this helps