Haven't caught season 3... but with plural famililes... yeah... it's TOTALLY common.
One thing you have to remember is that they're ALL MOM. Just like adoptive parents love their children, the non-biological moms have raised and loved each child as their own, as well. It's reeeeeally common for certain parents to get along (personality wise) with certain kids more than others. It's not that you love one of your children less, it's that you CLICK with certain kids more than other kids.
Similarly, you know how certain parents in monogamous families butt heads with ONE of their kids all the time, but not the others? Same thing happens in plural families. But in plural families one of the BENEFITS is that the kid who pushes all your buttons, doesn't push a single button of one of their other moms.
I love ADHD kids. By and large, they are RELAXING for me. My SIL, otoh, is punctual/ precise/ linear. HER adhd kids drive. her. up. the. wall. So they can come stay with Aunt R. for a few weeks every summer and we have a blast and my SIL gets a break. They are EXHAUSTING for her, but RELAXING for me. It's a "click" thing. I love ALL my neices and nephews... but the ADHD ones and I REALLY click. Same token, the tomboys (not ADHD) and I click. It's JUST about personality. We either think the same, or we enjoy the same stuff.
In a plural family, one of the HUGE benefits, is that there is almost always that 'connection' with one of your moms. You don't destroy the relationship with your biological mother by fighting all the time... you have a GREAT relationship with your biological mother... because you spend most of your time with one of your moms that you don't drive crazy. And because you're not driving your bio mom crazy, you're able to have a loving relationship with HER as well, instead of constantly fighting with her. Same token, you're not allowed to disrespect your bio-mom, because your other moms won't tolerate it. So you end up (usually) with happy moms and happy kids.
One thing to keep in mind, as well, is that all of the moms on this show GREW UP in polygamy. They had moms they were closer to than others, and moms they fought with more than others. They KNOW that one of their bio kids living with one of the other moms DOESN'T MEAN that they don't love their bio kid to bits, or that their bio kid doesn't love them. It's a personality thing OR a logistics thing. (Like often, when one mom has a new baby, the rest of her bio kids spread out with their other moms, so no one gets all exhausted).
It's also pretty fluid. Sometimes one mom is better with toddlers and another mom is better with teens and another mom is better with homework or teaching and another mom is better with sports and another mom is better with cooking and another mom is better with emotions (there can be 20 things to be better with, and only 2, 3, 4 moms, ... just like we ALL are better with/enjoy some aspects of life/parenting more than others)... The kids naturally gravitate toward the parent that is "better" with them at a particular stage or with their particular interests.
MAJOR PERK. & Yes. TOTALLY normal, culturally speaking.