If I remember correctly, Flex 1.5 uses Actionscript 2.0 while later versions of Flex use Actionscript 3.0, so I think your answer is almost certainly 'NO'.
You'll need to rewrite the code. I'm sure Alex Harui will have the definitive answer.
Thanks for your early response....
Can you send Alex Harui contact mail so that I will approach him through mail. Its high time for me to get resolve this issue in my current project. Kindly help me pauland.
I am not part of Adobe and do not have the email details for Alex Harui. It is up to him if he wishes to contribute to the discussion.
I think your question has been asked in other threads in the past. Don't ask me where.
Essentially Flex1.5 is an obsolete technology and I think you are looking at a substantial investment to make your project usable with Flex 3.0 (also essentially obsolete).
I would look at keeping Flex1.5 by using obsolete server software, perhaps a dedicated server machine, or moving forward to Flex 4.6/8 or another technology entirely.
You do not have an easy way forward.
What do you mean by "run 1.5 in 3.0"? Do you want to use Flex 3 to load a 1.5 SWF? Or upgrade the 1.5 project to use 3.0?
It might be possible for some 1.5 SWFs to be loaded by a 3.0 SWF. I've never tried it.
Paul is right that Flex 3 uses ActionScript 3 and Flex 1.5 uses ActionScript 2. Depending on how you wrote your code, it might be practical to port it. After all, Flex 3 was written by porting Flex 1.5 and Flex 2 from ActionScript 2 to ActionScript 3. It wasn't fun at all, but in your cause, you may not have to port every line. Prototype inheritance still supposedly works in ActionScript 3.