ITA agree with the PPs. I actually think I would refuse to spend time with your parents-in-law until they can show your boys fair attention. How must your poor children feel with their grandparents fussing over their other grandchildren but not them? You don't have to put them, or yourself, through that. If your MIL and FIL can't play fair, that's tough, and they miss out. They are acting horribly and they deserve to have someone pull them up on it.
I'm so sorry you are hurting. If it helps, I think three boys would be very precious indeed, and I'm sure you'll have your own little girl to dress is sparkly things very soon.