After I boot from my Ubuntu 10.10 cd, how can I access my files in sda3?
There's no /dev/sda3 folder here.. (the partition is ext4)
Your files are probably in one of the physically installed Hard Disk Drives which must be mounted in order to gain access to your data.
In Nautilus (or whatever file browser you use) try gaining access to the physically installed disks in the "Devices" section of the left side bar. Simply mount it and navigate to the "home" folder of the disk, which -of course- won't be your live session's home folder unless you mark it so.
Please inform if you succeed and good luck!
AS mentioned previously, those files are on the partitions that are not mounted by default. You could mount those partitions by clicking on them on the left panel of Nautilus. However, if this is to recover data, I would STRONGLY recommend that you mount those partitions in ReadOnly mode. You should be able to do that using the mount command at the command prompt. enter the command:
to see the parameters of it.