Hidden Codes
First, enable the cheat menu. At the title screen, quickly enter the following:

Up, Up, Down, Down, Left, Left, Right, Right

You'll hear a confirmation sound if done quickly enough. Press Start to access the main menu, and then press:

Block + Run

You'll access the cheat menu.

When you get to the character selection screen, highlight "Hidden", and put in the code you desire.
Code Effect:

Hold L + R, and press Up, Left, A to play as Goro;
Hold L + R, and press Up, Up, Left, Left, Left, A to play as Noob Saibot;
Hold L + R, and press Up, Up, Up, Up, Left, Left, Left, Left, A to play as Sektor.

=P

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There is one other way to play as all three of the hidden fighters. Meat is still in there too. Anyway, the codes on the select screen are the same, but you don't have to eneter the Cheat code menu unless you wanted, just follow the first step below, and then use Goddess's Select Screen Codes to access them.

To Play As Sektor: Beat the game with Cyrax. Choose Hidden with Block and Run and keep holding them. Move up to Cyrax and press A

To Play As Goro: Beat the game with Shinnok. Choose Hidden with Block and Run and keep holding them. Move to Shinnok, and press A.

To Play As Noob Saibot: Little different here. Beat the game with everyone. Choose Hidden with Block and Run and keep holding them. Move to Reiko and press A.

To Play As Meat: Complete the Group mode in a 2 Player game.

Those are the other ways to play as the hidden fighters. I still like Goddess's way better.