Worked with a mom with triplets in the last six months -- since babies were born very early and NPO at first, mom got a headstart by pumping q 2 hours, for 20 ea. side.  And having read that it was frequency of feeds more than duration that increased supply, she pumped  one side at a time, 10 min. ea., then went back and did another 10 min. ea.  By the time the babies could be given anything (tube feedings), there was enough stored colostrum, then milk, 
that the babies never received any other concoctions.

One problem with the "rotation" system is that you can't be sure babies 
will all be hungry in rotation, or at the same time.  So what this mom
planned to do was feeding each baby a bottle every third feed for 
that baby.  I.e., A got bottles 1st-4th-7th, B got bottles 2d, 5th, 8th, C got
 bottles 3d, 6th & 9th.   And every time she'd fed the second of two
 babies, she'd pump an additional 10 min. ea. side.  Of course, these 
kiddies weren't going to breast all that much, all that soon, as they were 
30 weekers.  And in NICU there is lots of help with diaper changes etc.  I 
don't know what actually worked once she went home.

You remind us that your mom has a three and a six year old already --
What it is paramount that she have is LOTS OF HELP.   Friends and 
family and paid help if necessary, to be available to the other children for 
special attention, and for cooking and cleaning and shopping and laundry,
so this mom can get as much rest and nurturing she needs to feed three 
babies and pump and recover herself, and maybe even have some time to
 relate to her partner and her older children.

Best of luck to her -- Chanita, San Francisco