You don't invite anyone, your host invites them. If your mom is on good terms with your stepmother and MIL, and vice versa I think it's a nice gesture for them to be invited to each other's events. Let the hosts figure it out for themselves, don't get in the middle.
Do you think maybe she simply asking whom should be on the list for each different shower? I know it is typical around here for the first sentence of a reply to remind people that they don't host their own shower or have any say in how their shower goes...but your "reminder" here is kind of a stretch.
Again, not something she needs to worry about. Part of being a host is determining who's invited. I don't see it as something she needs to worry about. If the three moms are all cool with each other, they'll probably invite each other without input from the OP. If they're not ok with having the ex-wife/current wife/ MIL around, they won't; and if that's the case, the OP's input wouldn't matter anyway, KWIM?
I'm sorry, I have never heard that that is part of a host's job. The host should determine how many people the accommodate, but pick who's invited? How often does the host know all the people attending?
And what if her mom invites the MIL and Step Mom but the MIL doesn't? Then she's the common denominator in some family drama.
I often read here (and agree) that if you have multiple showers it is in poor taste to invite someone to more than one--but many people feel that exception is made for the mothers of the MTB and FTB (or bride & groom), so she simply could have been checking to see if that was an okay thing to do.