Uchiyama-san let me know that ext3 file system is available on F3RP61.
I serached for the information to create ext3 file system on Linux.
A web page says that the mke2fs command accepts "-j" option to make the command create a ext3 file system.
The following is the log messages when I tried it on F3RP61 to create ext3 file system on User Sram.

-bash-3.2# mke2fs -j /dev/mtdblock4
mke2fs 1.39 (29-May-2006)
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
1024 inodes, 4096 blocks
204 blocks (4.98%) reserved for the super user
First data block=1
Maximum filesystem blocks=4194304
1 block group
8192 blocks per group, 8192 fragments per group
1024 inodes per group

Writing inode tables: done
Creating journal (1024 blocks): done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 29 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.
-bash-3.2# mount /dev/mtdblock4 /mnt/sram
-bash-3.2# cd /mnt/sram
-bash-3.2# ls