Decorate Tables
Assume $r\geqslant b\geqslant g$. Then, if $r\geqslant 2(b+g)$, the answer is $b+g$ because the best you can do is to use two red balloons for each table.
Conversely, if $r<2b+2g$, then you can check by induction that the answer is the integer part of $(b+r+g)/3$.