I was faced with the same issue--I have 4 boys :) One thing we thought of doing was to have a "welcome baby" celebration. Instead of doing the shower before, have everyone come over to meet the baby after. Even if you tell people to not bring gifts, they do because I think everyone likes to buy baby stuff :) It is nice to get new outfits for number three and it's fun to see everyone "oogle" over the new baby. One suggestion would be to involve the other two boys as much as age appropriate in the party planning so they feel a part of the celebration also.
I also had some friends that really wanted to do a shower for me, so they did a diaper shower. You always need diapers! Everyone bought disposable diapers, but then they wrote messages with a sharpy pen on the front of each diaper, i.e., "I love you Mommy" or "To pee or not to pee, that is the question." When you are changing diapers at 3 AM it's pretty funny to read the messages.