Hi, Lulu, you sound exhausted - 3 kids is pretty full on! Your little one is .....normal! doesn’t mean it’s not really tiring. In terms of not enough wet nappies - does she drink much water? the answer is not breastmilk or formula at this age - she should be having some water to drink in a cup as well. Is she a good eater? Does she eat a range of food including meat?
In terms of sleep there are a couple of ways to skin that cat. My preferred is the route of least resistance! Co-sleeping can be very helpful. Controlled comforting may work (or may not) and you have to be very prepared for some *very* bad nights.