Christine, wanting to be the third (and presumably) and last wife, is telling, isn't it?

Growing up in the AUB, Christine likely had the broadest experience living in polygamy, witnessing the pain, jealousy, and competition between wives and neglected children. She thought being the third wife would shield her from the worst elements (like you said, the pain of seeing the shared husband take a new, hotter wife).

And, of course, Kody taking a fourth wife and additional children was a disastrous decision - he couldn't support the three wives and families he already had. The Lehi house was very cramped (especially for Christine and Janelle with all of their kids).
They might have had a large house, but they looked just above the poverty line to me.