The latest version of Flash for now is CS 6 - i use old one CS5.5, but CS6 is better and have some new abilities. It is better to look for CS6.
Yes new project do not open in CS3, but if you will have problems with that (can not download new version - limited in internet or some time limits for it) - try to play with some stuff on flashkit. There are a lot of good stuff even for Macromedia flash MX and Flash 5 (The oldest version that was more than 7-9 years ago).
A lot of people on forum use CS4 and older versions - i think even 30% use CS6. One day i plan to use CS6 too or CS7 if they make it
But now it is a problem for me to install and looking for updates of my version of CS5.5 flash - becouse i have limits with time - a lot of business trips on work and etc
New functions in CS6 is sprite list and some stuff fore internet - so i do not need it for now
But some day i think it can be usefull.