It might be easier to do in Flex 4.
it might, so is it possible? (im asking about Flex 4 )
Probably, but I would think it would look jumpy as the buttons come and go.
I'd probably disable instead.
There are several examples around relating to scrollers and enabling/disabling parts of the scroller. You could create custom skins for the scroller buttons that disable/hide the buttons but i'm not sure if it is really worth the effort.
For instance if the thumb is at position zero and you hide the top/left button would you move the thumb to the position where the button used to be or just leave a 'blank' space, I think both these options are going to lead to a not so pretty visual effect.
So what you want to do is possible probably the best way is through the skin parts of the scroller.
http://blog.flexexamples.com/ Peter's blog has a couple of examples of scroller skinning.
Also there are a few threads in the forum where people have discussed manipulating the scroller.
I have scrollbar skinned, and im not using track or thumb, just 2 arrows. I hope that visibility change (not includeInLayout) would have same size. It's just client request, not mine