If one eighth of the pizza falls, that leaves seven eights of the pizza to be shared:
\(1\ (whole\ pizza) -\frac{1}{8}=\frac{7}{8}\)
If the 7/8 pizza is shared between three people, then you have to divide 7/8 by 3. We know that dividing a fraction by 3 is the same as multiplying by 1/3:
\(\frac{7}{8} \div 3\\\\\frac{7}{8}*\frac{1}{3}\\\\\frac{7*1}{8*3}\\\\frac{7}{24}\)
Therefore, each person gets 7/24 of the whole pizza.