Remaping PS3 Controler as 3rd Player in FBA

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I'm making a two player arcade using the IPAC2 board mapped as a keyboard for two players.
IT works. I can play two player games in all of the platforms I chose to use.

Sidenote: In the main menu, only the first controller is able to navigate. I suppose it should work this way.

I tried hooking up a ps3 ontroller to use as a 3rd player control for some Arcade Games as Cadillacs and Dinosaurs, TMNT and others.

The ps3 controller is recognized and it works in Main Menu.
Unfortunately, retro Pie assigns it as a second player control.
It turns out that both the second joystick and the ps3 controller can control the second player simultaneously.
Can this be avoided in some way?

Before we used to play with friends in 3 player games using Xbox controllers.
I assume that the problem comes from the IPAC being recognized as one device.

Any suggestions?
Thanks in advance.

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I might have figured this one out.
It came to mi\y mind that I should simply map the controls inside the retroarch.cfg file.
Here is the code I have written for the PS3 controller. I
I took thr button numbering from the : retroarch-joypads/SonyPLAYSTATION(R)3Controller.cfg

---

input_player3_joypad_index = 2 (This maps the ps3 controller as 3rd?)
input_player3_select_btn = "0"
input_player3_start_btn = "3"
input_player3_left = "7"
input_player3_right = "5"
input_player3_up = "4"
input_player3_down = "6"
input_player3_a_btn = "13"
input_player3_b_btn = "14"
input_player3_x_btn = "12"
input_player3_y_btn = "15"
input_player3_l_btn = "10"
input_player3_r_btn = "11"
input_player3_l2 = "8"
input_player3_r2 = "9"
input_player3_l3 = "1"
input_player3_r3 = "2"

#Axis for RetroArch D-Pad.
input_player3_l_x_plus_axis = "+0"
input_player3_l_x_minus_axis = "-0"
input_player3_l_y_plus_axis = "+1"
input_player3_l_y_minus_axis = "-1"
input_player3_r_x_plus_axis = "+2"
input_player3_r_x_minus_axis = "-2"
input_player3_r_y_plus_axis = "+3"
input_player3_r_y_minus_axis = "-3"

#hotkeys
input_enable_hotkey_btn = "0"
input_exit_emulator_btn = "3"
input_save_state_btn = "11"
input_load_state_btn = "10"
input_state_slot_increase_btn = "5"
input_state_slot_decrease_btn = "7"
input_menu_toggle_btn = "nul"
input_reset_btn = "nul"

---

I will have the opportunity to test this tomorrow.
Does this seem right?

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This one is resolved.
For everyone having the same issue.
In order to enable 3rd and 4th player through joysticks, you have to first install the ps3 or Xbox controller drivers through the retro pie setup menu.
Do not input any specific button configuration inside the per console config files.