Quoting ♥Mama Constantine:" Or to anyone else that could help: I'm babysitting a 3 year old, who is 5 days younger than DD#1, and ... [snip!] ... so I'm having to rock them while they cry. And how can I keep two very loud 3 year old's quiet while the babies sleep? TIA."
That's brutal when they do that, but the one thing I learned when I had my twins is that sometimes there's going to be one crying while you calm the other one. It totally sucks, but it's just how it goes. There's not much you can do when they both start wailing. I used to lay DD and DS down side by side, make sure they're changed and such, and then begin to try and feed them.
Another life saver for me were baby wraps...My bestie (Because I could hold them both at the same time, and if it was just them both being needy, it would calm them down)
In this case, unless you have a twin wrap (Which I'm guessing you don't) You may just have to put one down and let them cry while you take care of the other one. (One of the reasons twins are such good self soothers is because they get used to Mom not always having time to hold them both) That's going to be tough, since they're not twins and are used to getting the attention they want. Your other option is to go lie down on a bed with both of them, and try and settle them down that way. Which may work, but then becomes a pain to move them to their seperate sleeping areas.
As for the three year olds, all I can say is that while you're calming the babes down, a TV is a lifesaver.