The bios will look initially at the MBR of the first disc (hd0) but this can point to the first partition (hd0,0) or another partition say (hd0,6) where the main part of grub resides. In my own case my MBR on hd0 must point to hd2,6 (I have three drives as well) where I have Debian. I have Mint on hd2,5 which I can select from the grub menu.lst on the Debian install. Mint has its own menu.lst left there when it was the last install but it is never seen. I have always let the distro install grub to the mbr and it sorts itself out, recognising all the existing distros. I'm not sure but I think grub-install hd0 may do the same. The grub documentation is a bit bewildering (to me at any rate) but I expect Eric will explain it!
Tony