Thank you for the replies to the questions and the added information. It helps a lot in understanding what the situation is.
I can imagine how frustrating this situation must be for you. You are clearly loving parents and this third child is giving you a run for your money. Or your sleep, in any case!
It sounds as though you've ruled out pinworms, any neurological issues, or other medical problems in your own minds. Okay, then we're left with attachment issues or simple anxiety that's led to a behavior pattern that she's entrenched in (your theory). Your theory is a good one, given that developmentally she's on track, socially she's fine, etc.
There may be a problem of over-achieving developmentally that's causing some anxiety for her. By that I mean that she's the youngest of 3 and is most likely trying to compete with everything her sisters can do. She's coming out of that mommy stage, but that isn't the only manifestation of bright kids' frustration developmentally when they're trying to do what older siblings are doing more easily. And she's not fully cognitively formed to boot.
Much of this also requires you two to not make too big a deal out of it or it will become an attention getting tool, subconsciously a specialness about me that gets mom and dad's attention. So you have that to contend with as well.
We really are, then, looking for small things to do to break the behavior pattern and relax her anxiety. Brighter lights is a mixed blessing: it can become a long term need for her and, given she's in your room, it may be tough when she has to sleep with her sisters and they're not used to bright lights. So consider if you want the light to begin with and if it will be counterproductive long term.
The cot is fine. Some parents I've worked with had given up and let the child sleep in bed with them. This too often forced one of the parents to sleep elsewhere as the child would thrash about and kick in the night. They did this just to get some sleep... So, you're not alone and it can be tough. A cot, if you can make it work is a great help here.
So, there are two times in the evening we're trying to work on: bed time and sleep time.
I've had good success with the following strategy: parents (usually mommy) has one of her soft sweatshirts, scarves, etc. that she wears for a while. Each evening before bed mommy (or daddy) takes off the scarf and kisses it, the daughter kisses it, and they both hug the scarf and say how much they love each other and that the scarf is a special thing between them to help them feel their love. Then the daughter is presented the scarf to take to the cot with her and keep so that when she wakes up, she can snuggle with the scarf and know how much mommy and daddy love her and how special she is. The comfort is transferred to the scarf rather than to your making an appearance.
At bedtime, the technique I encourage the most is letting 3 year olds regress. This means holding them in your lap and singing songs you used to when they were babies. Talk baby talk with them and see how young they would like to get. Youngest children are trying so hard to be as big as their sisters that giving developmental relief, letting them regress, is often a great comfort to them. Make up baby talk sing songs if you don't remember anything from then. Bring out the picture books from when she was younger.
All this will take some time, but you're right in that stage where you can't do more: the other kids will wake up and she's not developmentally where they are. So you pretty much have to keep her with you as there's no other room.
Okay, I wish you the very best!
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