Hi, I've recently installed gentoo in my system, and I actually assigned 4 Partitions to gentoo in a SATA Hardrive:

/dev/sda2 - /boot
/dev/sda3 - /
/dev/sda5 - /home
/dev/sda6 - swap

everything was going fine after install/configure grub, 'cause after restart my system and select Gentoo from the boot list, when the boot process proceeds to mount the filesystems appears a message saying that the block device /dev/sda3 is not a valid root device and asks me to specify a device to boot, 'cause the the root block device it's unespecified or not detected, when I actually configured the grub.conf file with the next (according to the Gentoo installation guide):
kernel /kernel-2.6.11-gentoo-r3 root=/dev/ram0 init=/linuxrc ramdisk=8192 real_root=/dev/sda3

'cause I've been trying with make some changes at the grub configuration file, I've checked the fstab file to find if something it's wrong, and until now nothing haven't worked, I would really appreciate some help, thanks.

Try adding udev at the end;
kernel /kernel-2.6.11-gentoo-r3 root=/dev/ram0 init=/linuxrc ramdisk=8192 real_root=/dev/sda3 udev

I added udev at the end of the line but the problem persists, I don't really know if the problem it's from Grub configuration file, or something from the instalation.

Did you emerge udev?Here is the guide;
http://www.gentoo.org/doc/en/udev-guide.xml
other than that I would try to compile my own kernel,I have seen problems over at gentoo forum's where that has worked for others,can't hurt good luck