Portable MK has discovered another finishing move in the GameBoy version of MK3, after 18 years.

Simply hold PUNCH and press DOWN four times in the third round and the defeated opponent will be brought back to life with a small amount of health and the word "MERCY" will be displayed.

People have known for a long time that "Mercy" and "Friendship" were text strings in the hex code of the ROM, but it wasn't known if it was possible to perform them until now.

As for the "Friendships" it has been confirmed that they are NOT possible. There are no additional graphics for them. There ARE special input combinations to perform them, but they are blocked out of the normal program, which indicates that at one point the developers were considering including them.

Source:
http://pmk.mortalkombatonline.com/