Congratulations shrub. Why should you only have 2 children? If you guys want and can care for a third (which is obviously the case) then it's great that you're having another.
My mother-in-law was dissapointed that we were having a 2nd. I can't understand it to this day, more than 3 years after we told her. When we told her, her face fell, and the first thing she said was "I just got my Christmas tree skirt embroidered with all the grandkids' names." (There were 8 of them at the time.) "Now I'll have to take it back again." Then she realised what she was doing, and hugged me and my husband and said "oh, that's great". I was fuming.
She had 5 children of her own, so does she expect her children all just to produce only one child so that she doesn't have too many grandchildren? And it was never a secret that we planning to have more than one child.
Now, of course, she loves my youngest every bit as much as her other grandchildren. In fact, even before she was born she did. My husband died when I was 11 weeks pregnant and my MIL was extremely concerned and worried about how the stress would affect the baby.
So I'm sure that once the shock wears off (not that there shoud have been any shock), the grandparents will be as thrilled as you are, but it sure does hurt when they aren't as enthusiastic as you'd hope at first.