Answer
Three rubber bands are required.
Work Step by Step
Let $F$ be the force provided by one rubber band. With the information in the question, we can set up a force equation;
$2F = ma$
$F = \frac{ma}{2}$
Let $n$ be the required number of rubber bands;
$nF = (\frac{m}{2})(3a)$
$(n)(\frac{ma}{2}) = \frac{3ma}{2}$
$n = 3$
Three rubber bands are required.
