I think the real issue here is not that people genuinely believe that small children should have a choice about where they sleep, and whether they prefer to sleep alone or with a parent. It is that people see co-sleeping as somehow deviating from the norm, and therefore, they think it can only be justified if the child has actively chosen it.
Yep.
I wouldn't worry at 3 but they definitely do need to transition to their own bed at some point in the next year or so
Why specifically? What is different at four (or five) to make it a problem that isn't a problem at three?
I'm not being goady/argumentative, I'm wondering what the thought process or reasoning is. Because for example, if a baby is using a bottle aged 9 months, that is OK, but at 18 months they really ought to have stopped, and the reason is that 18 month olds are less reliant on milk, so the cost (to their teeth) and benefit (drinking enough milk) is reversed by 18 months. I can't think of anything like that which applies to sleep and ages 3 / 4 / 5.
with many ASD children is it's very hard to break habits...so by building the habit of sleep together now it may be very very hard to break...It won't be easier in a year or two it will be harder[...] your better to lie in her bed or on her floor now if that's your plan.
I think this is faulty logic - things don't automatically get harder the longer you leave them, in fact with children often things get easier as they develop more and are less reliant on the thing. I think where this comes from is the idea that for example if you have a little baby aged 6/7 months, it's "easier" to sleep train them at that age than it is to let them keep co-sleeping and then try to kick them out at age 2. But that's not because the 2 year old has been co-sleeping longer so the habit is harder to break. It's just because a 2 year old is much more verbal, more persistent and can walk so you get more of a guilt trip, whereas a 6mo "just" cries and then gives up fairly quickly because they can't do anything about it.
But if you think about it, if you decided to co-sleep for 6 months age 2 and then try to move them out it will probably be just as hard, because 2 year olds generally don't like change and being told what to do because they are in that defiant toddler phase, plus many 2 year olds are still quite reliant on parental comfort or other sorts of comfort like dummies, thumb sucking, a comfort toy/blanket etc, and they don't have a very developed sense of time, so a parent leaving for the night might as well be leaving forever in their mind. In that case (thinking of a NT child here for simplicity) waiting a couple of years might actually make things easier, because 4 year olds are that bit more independent, the understand so much more, are less reliant on those props that they needed when younger, and actually many prefer to sleep alone - mine both did, even though we had coslept until they were 2. (If this is a bad example, think of something like potty training - it is often easier the older the child is as they are more verbal, better at recognising signals, more desire to be independent etc.)
To bring it back to the OP, they have presumably been cosleeping for 3 years already, so it is too late to just not start the habit in the first place, and waiting another year or two is unlikely to make much difference in the area of how entrenched the habit is. But, it probably will make a difference in things like verbal reasoning, self regulation, sense of time, ability to cope with change. These are all things which are often delayed with ASD so they will probably take longer to come than with a NT child, but that does not mean they will not develop at all.