When shrinking a ext2/3 file system it is better to unmount it first. It is not explicitly stated in the resize2fs(8) man page but implied not to support on-line shrinking; only on-line extension.
+1 on syg00's comment.
- Backup filesystem! There's no guarantee that data won't be lost - it is a high risk especially when shrinking.
- Unmount filesystem
- Shrink filesystem (resize2fs specifying the new size)
- Shrink volume to a size equal or above the filesystem (lvreduce)
Another option that may
recover the situation is by running the following (use -f if you need to) and after do a filesystem check:
resize2fs -p /dev/vg1/lv3 512M
By shrinking the volume first, part of the underlying filesystem has been destroyed. The only option might be to rebuild the filesystem, if syg00's idea of extending the volume again doesn't work.