It depends. Let's imagine forA, and forB are expressions that evaluate to futures of Ints. This runs them in serial:

   for {
     a <- forA
     b <- forB
   } yield a + b

This runs them in parallel:

   val fa: Future[Int] = forA
   val fb: Future[Int] = forB

   for {
     a <- fa
     b <- fb
   } yield a + b

(I dropped forC from your example as it has a dependency on the result for forA, and thus can't run in parallel.)