I think it is fine to have a baby shower for your third baby - especially since you don't have anything anymore. Since your friend offered to give you a baby shower, take her up on her gracious offer!
Why didn't you invite any of your MIL's family for your baby shower for your son? I would skip the "meet the baby party," accept your friend's baby shower offer, and invite your MIL's side of the family. Why not? More gifts for you/your baby! Congrats!
ADDITION: Okay, I just read the posts, and just about everyone thinks more than one shower is tacky. I understand where they are coming from by saying a shower is for a first time mom, but I also see the other side, which is - just like J9 said, every baby deserves to be celebrated, and I totally agree. People who don't want to come to your shower (because they think it's tacky) won't come, and the people who want to come to help you celebrate another baby's life will come. The people who come do not have to spend a lot of money on your gift. I'd spend $10-15 on a baby shower gift for a good friend - and in exchange, I get to dress up a little and go to a party where there would hopefully/probably be yummy food, and maybe even mimosas? (I know, the guest of honor can't drink, but a mimosa brunch would be lovely, and I'm sure the guest of honor would be happy drinking orange juice)! Life is too short to not celebrate the joys of life, and this is a perfect "excuse" for a party. Again, no one has to break the bank over a baby gift. Your close friends will come, I'm sure. Cheers!