I've vista and Ubuntu installed in my system.
now I want to install fedora 13,so what do I need to add to Grub to be able to tri-boot.
my configuration.
vista is in /dev/sda1 partition.
Ubuntu is on /dev/sda3 partition.
I would like to install fedora on /dev/sda2 partition.

Go ahead and install Fedora. It will install it's own boot loader into the MBR. Once complete, you will reboot and see options for Fedora and Other (Vista). There will be no entry for Ubuntu as Fedora seems to be lacking in its ability to detect other Linux installations. Now, to get Ubuntu back there are a few options available. The easiest, in my opinion, is to reinstall Ubuntu's bootloader using the Ubuntu installation CD (assuming Ubuntu is using Grub2). If the Ubuntu bootloader doesn't recognize the Fedora installation, boot Ubuntu and run sudo update-grub from a terminal. This should then give you all three OS choices.

If you're using a slightly older release of Ubuntu, one that uses legacy grub, you can simply add an Ubuntu menu item to the Fedora bootloader. Copy and paste the info from Ubuntu's /boot/grub/grub.conf.

But as Fedora updates it's kernels will Ubuntu reflect that in the Boot splash screen ?

In my opinion it will not .
If sudo update-grub works than you have to run it again.

Ubuntu will not, if using grub2, reflect updated Fedora kernels in the menu until you boot into Ubuntu and run sudo update-grub.

I will not install Fedora's boot loader and after installing fedora I will run sudo update-grub command on Ubuntu,will that be enough?

You do not tell us with version of UBUNTU you are running .
If it is UBuntu 10.4 you are running GRUB 2 if you do not change it .
In that case yes running update-grub in UBUNTU is enough until Fedora get a new kernel than you have to run it again in UBUNTU .
Older versions of UBUNTU are running GRUB 1 in that case ad it to UBUNTU's menu.lst
Do not know exactly UBUNTU switch to GRUB 2

yes I'm running Ubuntu 10.04 and would like to install Fedora 13

Not sure how to set it up in grub2 (I'll stick with grub legacy for a while longer), but I'd suggest installing Fedora's grub to the Fedora partition and setting Ubuntu's grub.cfg file to chainload Fedora's grub. That way you won't have to go back to Ubuntu just to fix your grub configuration file every time you update the Fedora kernel

we can't chainload in Grub2.

That would make booting Windoze very difficult - of course you can chainload.
Fedora installed as RockDoctor suggests works fine.

Hi!
Since 2 years I have triple-boot. VISTA was installed; I than installed
Ubuntu (GRUB 2) and later on Fedora. If you install Fedora in a tripleboot as the third operating system, you have to install the Fedora-Grub legacy into the partition sda2 (in your case, because you only have one partition for Fedora); during the Fedora-installation you must change the device in which the bootloader should be installed: sda2 instead of sda. After you 've success with installing Fedora, you must run an grub2-update as root in Ubuntu: (Terminal)
Than a new grub.cfg is created with all recognized operating systems; after rebooting you should see something like
GNU-GRUB V 1.98...
...Ubuntu 10.04 kernel..
...
...VISTA
...VISTA Recovery
...Fedora 2.6.34...
...!
Good luck!
Alfredo