To find the common elements from multiple vectors, you can use the intersect function from the sets base R package.
The basic syntax is given below:
vectors (of the same mode) containing a sequence of items (conceptually) with no duplicate values.
Intersect will discard any duplicated values in the arguments, and they apply as.vector to their arguments (and so, in particular, coerce factors to character vectors).
In your case:
a <- c(1,3,5,7,9)
b <- c(3,6,8,9,10)
c <- c(2,3,4,5,7,9)
 3 9
 3 9