We've had the extremely asynchronous non stop talking co-sleeper (or rather co-waker) up till four, the one with feeding/issues (not entirely resolved yet but infinitely better after 18 months incredibly hard work) and the extended nurser with major medical issues. The feeding issues I'd say were the most frustrating.
I can imagine that having all three rolled into one is exhausting to a point that others experiences cannot compare.
I usually go to bed with my children around eight because I have usually been completely wiped by late afternoon. I LOVE LOVE LOVE it when for some reason I get to have an afternoon nap with DS 1. Bliss.
You say you are young enough to wait and see and having them close enough for them to be playmates (I'd say between 4 and 5 is probably the outer limit for that) is not a concern for you. So, wait and see! It may all suddenly fall into place,
I cannot recall whether your little one is reading yet? If he is, he surely hasn't got the stamina for the kind of reading yet that fills his need for mental stimulation without constant interaction with a receptive grown up. I recall DH saying what an eye opener it was when our oldest had actually shut up for almost half an hour for the first time ever because he was making his way through his first chapter books "I realized just how exhausting the last five years have been and why, and realized that things will get better...".
Now some of that uncontrollable mental energy goes into picking on his little sister, I'm sorry to say...but never forget that parents do have some control over how well siblings get along (siblings without rivalry, while rather dated, had helpful ideas for me, and made me realize how parents can actually create the rivalry).
I sometimes feel a bit sorry for my DD, sandwiched between her two brothers (I read the same article Bostonian read, but somehow the two daughters thing didn't work out for us either, some glitch in the ordering of the universe I expect) but on the other hand, you've never seen such a tough little girl!
