I'd noticed that Flex Builder 3 was kind of running like a dog on OS X, and crashing at least three times a day when working with large projects. My laptop is plenty powerful enough, so I figured either (a) the software is buggy, or (b) the config settings for OS X were not right.

I opened the Flex Builder 3.app file and found the FlexBuilder.ini file, which I modified to the following settings. The memory seemed to be capped at 512 MB from what I could see, and since I am usually running FB3 almost by itself during my day job I figured I could be a little more generous with the memory.

Here's my config file. I pretty much multiplied all the settings by two if they were small, and added 256 MB to the JVM Xmx to allow a higher memory ceiling. Since the change, FB3 seems much more responsive and has not crashed on me yet (knock on wood).

-vmargs
-Xdock:icon=../Resources/flexbuilder.icns
-Xdock:name=Flex Builder
-XstartOnFirstThread
-Xms256m
-Xmx768m
-XX:MaxPermSize=512m
-XX:PermSize=128m
-Dorg.eclipse.swt.internal.carbon.smallFonts
-Dorg.eclipse.swt.internal.carbon.noFocusRing

On a side note, when I am using FB3 with only one or two small projects open, it runs great; the problem seems to be related to huge work-related projects only. This is what makes me think that the memory settings were the underlying issue.