I have a mixed SL / Leopard environment and never noticed this. I will check this evening when I get home. Have you applied all your updates to each OS? I know there were quite a few Apple Remote Desktop updates over the past few months and since the same technology is used for VNC/ screen sharing / ARD, not being up to date on each OS may create issues. Also you can create a shortcut to screen sharing on your dock by dragging the screen sharing app from Applications to your dock. You can then browse all the available screen sharing on your network using bonjour or simply enter the ip address of the machine you want to screenshare in to. Lastly, don't forget to enable screen share and vnc on every machine and make sure there is a vnc password (vnc uses a separate from the screen sharing which requires an Apple login to the machine you are trying to screen share). Make sure the account you want to use exists on all 3 machines. You will need to log in to screen share. If you are connected as "guest" you cannot screen share, so you could disconnect and "connect as" a legit account on the machine you are trying to control.
hope this helps...