Answer
$BF_3+3H_2O --> 3HF + H_3BO_3$
Work Step by Step
When balancing chemical reactions, it is often best to use a "guess and check" method. For example, if you see that there are 8 hydrogen atoms on one side and 4 hydrogen atoms on the reactants side, multiply the reactant with the 4 hydrogen atoms by 2. Then, count the number of atoms of each element on each side. Continue this process until you obtain an equation that is completely balanced. This gives:
$BF_3+3H_2O --> 3HF + H_3BO_3$
