It can take a long time.  Remember that new cats present a very
stressful situation for you older cat.  Cats exist in a social order
and work these things out themselves.  When all the cats are together
it's important that they figure things out on their own, there may be
some slapping, pouncing, hissing and growling - this is perfectly
normal behavior.  You are there to be the ref. in case things get out
of control - scrathcing and biting.  You can use a spray water bottle
for this, but avoid the eyes.  It's also important not to touch the
cats or show attention during this phase as it can create jealously.
Try to do things that will be fun for all of them such as: string, cat
dancers...things like that.  It will take a while but everyone will
come around.  We have two cats in a similiar situation and now they are
best friends.  You just have to give it time and NOT rush things.  At
worst, they will learn to tolerate each other's existance and learn how
to live with each other.

Also, keep in mind that your older cat is "top cat" and now you bring
two strangers into the house.  Cats can get very upset and hurt.

PJC