Consider the function.
\[
f(x)=\left\{\[b∣x2−7x+12∣a(7x−12−x2)b,x3,x=3,\]\right.
\]
where [x] denotes the greatest integer less than or equal to x. If S denotes the set of all ordered pairs (a, b) such that f(x) is continuous at x=3, then the number of elements in S is :