First off all, why would one want to downgrade an application? Flex 4 is backwards compatible with the older SDKs
Second, Spark components were not available in Flex3.
So your library project will probably be using s:SolidColorStroke
i.s.o mx:SolidColorStroke
, hence it cannot be found.
An SWC is basically a zip file that contains an .swf.
This is a binary (compiled) format over which you can have no influence.
Any SWC imported in your project should be compiled with the same SDK as your project, or specifically state the other allowed SDK versions, usually previous ones.
To make a long story short, you will need the sources of the library project (swc) to be able to recompile it with a 3.x SDK and make adjustments accordingly to fix the error.
You could also try to decompile the .swf in the .swc file, however I would not recommend this.
Cheers