I am in the camp that thinks it's tacky to have a baby shower for 2nd and 3rd babies. Also, clothes can be very cheap. Shop sales or nice consignment stores and you can get the basics for next to nothing. I assume you already have the expensive stuff and gear from your previous children.
I would just tell your sister in law that you really appreciate the offer, but since this is your third you'd prefer not to have a shower this time.
While I understand that your previous children have been slow nursers, every baby is different and this one may eat in 10 mins like mine. So I say just play it by ear. Even if the baby is another slow eater, you can always wait and have a "meet the baby" party whenever you're ready and you've gotten into a better groove of nursing, even if it's not right after he's born.
Edit: The point of a shower is literally to "shower" the mother with gifts (it's designed to give new moms a boost in preparing their homes for having a baby). I understand wanting to celebrate your baby, but gift giving is the central point of a shower. The same way many people would think it's tacky to have a wedding shower for your 3rd wedding, many people find it tacky to have a baby shower for a 3rd baby (you should have the basics, and by that time, should be able to provide anything else your child needs).
If somebody local, who can actually host the shower, offers to throw you one, that's okay. However, I would personally instruct her to say "no gifts" on the invitation. Surely some close friends and family will end up giving you things anyway, but that way you get to celebrate the baby without making people feel obligated to give a gift (or like you're trolling for gifts, which is what some showers can feel like).