smc

Let $a$ be the number of students that can only sing, $b$ can only dance, and $c$ can only act. Let $ab$ be the number of students that can sing and dance, $ac$ can sing and act, and $bc$ can dance and act. From the information given in the problem, $a+ab+b=29,b+bc+c=42,$ and $a+ac+c=65$. Adding these equations together, we get $2(a+b+c)+ab+bc+ac=136$. Since there are a total of $100$ students, $a+b+c+ab+bc+ac=100$. Subtracting these equations, we get $a+b+c=36$. Our answer is $ab+bc+ac=100-(a+b+c)=100-36=\boxed{64}$