I've had Metal SLug 3 for awhile now, played through it a buch of times, and I think the only slug I haven't seen was the ostrich. Does anybody no where it is?
In the underwater stage look for the pipes that lead you to different parts of the board. The first pipe leads you into the mini-sub segment the second to the Ostrich slug in a jumping segment
. The ostrich tunnel is the second tunnel that falls from the top of the screen when you blow up the tank barrier. God this is hard to explain, just take your time in the underwater stage and you will see them.