i too would be hesitant to think that it was the cosleeping that created the separation sleep issues with your DS3....

i coslept with my last two kids and they are such different sleepers to eachother. i really think that sleep is a human to human thing...its different for each and everyone of us ITMS.

i have found that DD3 will on most days go down ok with a boob to her bed during the day. but there are days when i can see that she just needs me to be there and i try to do that for her and we cuddle up and boob it up in bed for that sleep. it does mean a bit of TV for the older kids if need be. but we generally then are all ready for fun once that nap is done.