Okay, this is just my opinion...
Showers are meant to help people who are just starting out and have nothing. This being your third baby, you aren't starting out anymore. I wouldn't say it's really "tacky" but I think it might rub people the wrong way. Like, just another excuse to get more presents. I think if people will want to give you a gift, they won't need a shower to do it. They'll just give you a gift when the baby is born anyway.
I'd suggest just having a "Let's Celebrate J.'s 3rd Baby!" Luncheon. On the invitations, you could suggest that people bring items to be donated to a local pregnancy center.
I've had 5 kids, and I had 3 showers with my first baby. My church threw me a surprise shower with my 3rd, but the gifts were all hand-made.
I don't doubt that people want to wish you and your baby well as well as celebrate another beautiful baby being born. But I think it's just wiser to not invite people to a party in which they are required to bring a gift again. There are already so many events nowadays that we DO have to bring gifts to, that this seems excessive (even though I understand it's not your idea.)