Is it possible to install Win 7, Ubuntu, Debian as triple boot? If so, which one should I install first after Win 7? Will there be problem with GRUB? and Is it fine to install those two linux in logical/extended partition?
yes it's possible
you should install the windows 7 ,debian and in the last ubuntu . grub2 automatically detect the other os
1 - make sure it will not go beyond the 4 primary parition if it so then your pc will not boot up
2 - you can make extra partion by gparted tool but it will not more than 4 in case of primary partition while other extented parition is allowed
3 - after having the sufficient space for third linux distro install it grub2 automatically detect the other os .
4 - if any problem occur then reinstall grub2 of ubuntu
here is the video http://www.youtube.com/watch?v=8lod8sRb_6I
5 - now you are able to use all the three os installed in your pc
if the other distro is not show in grub2 menu then
you can updage ubuntu grub2 http://www.youtube.com/watch?v=WJDcyAncmRY
Yes it is definitely possible (I've done it before).
Windows 7 first so it doesn't overwrite GRUB, and then Ubuntu or Debian can be installed in whatever order you please. They will both install their version of GRUB onto your master boot record, but Ubuntu's will likely be a more recent version.
It is fine to use logical partitions, although you might have to chainload GRUB. As long as you aren't making any other partitions, you should be fine to install them all on primary partitions though.