so I'm lucky to have the newborns who scream constantly if they are not being held and/or nursed. When 2yo DD gets home from her temporary daycare, mayhem ensues. The twins want to eat every 1.5 hours or less, even overnight after cluster feeding all evening. We've had a couple decent nights where they did one 3-4 hr stretch, and I get my hopes up that we're turning a corner, but the next night I'll get 1/2 hr of sleep at a time. Please remind me this isn't forever before I send them back. I can barely handle the 2 by myself all day and I'm terrified for next month when I have all 3.
Re: 3 weeks in and going mental
I'm so sorry.
That time period is what I call newborn hell. Its a good thing they are so cute, otherwise I can see why you'd want to send them back. 
Sleep deprivation is the most cruel, terrible thing, but I promise you, before you know it, the first three months will be a huge blur and your twins will be giving you hugs and kisses! j
Hang in there. I can't imagine doing it with a toddler, but I am sure you are doing great! Just take it one day at a time and if the house goes to hell, just let it. There will be plenty of time for cleaning later. Try to rest whenever you can.
*HUGS!!*
I have newborns and a toddler as well. I'm FF and they are going 4 hours at a time with their formula. MY toddler is awful as well though. I'm hoping things get better for you.