just set ext3 support to M (module) in your .config and only rebuild your modules, not the kernel.
So "make modules" and "make modules_install" should do the trick. Be warned that "make modules_install" removes ALL your modules and replaces them with the newly built ones, so either make a backup or be sure all needed modules are in your .config file.
"modprobe modulename" (don't know the exact name of the ext3 module) should insert it at runtime. Too list all inserted modules try "lsmod"