im trying to get it to work on mame4all for psp, but when i load up mame for all it doesn't detect the game, i placed it in psp/game/mame4all/roms

Metal Slug 3 is the biggest Metal Slug by far, it takes FBA-XXX Pro on the Xbox a really long time to load it, I'd be suprised if the PSP can handle it. Is it definitely supported? Is it in mame4all's dat file?

Use MVSPSP to run metal slug 3 instead of Mame. mame is not stable on PSP.

thats a neo geo emulator if i am correct, now do i get neo-geo roms, or neo-geo cd roms for MVSPSP

Get the normal Neo-geo arcade rom.

For Neo Geo CD, you need to get the NGCDPSP emulator, which i am not recommending as the rom for NGCD is larger, not worth the space, plus lesser game since you won't get those "new" NG games like ROTD, SVC:C or Samurai V Special.

Seems there's also CPS1PSP and CPS2PSP, if I had a PSP, I'd want those too.

bios not found... so what now?

That means you need the NeoGeo bios. It's always a file called neogeo.zip, but the contents of that zip is different for every emulator depending on which version of MAME it uses. However, since I don't know what version of MAME this particular emulator uses, I don't know what files need to be inside said neogeo.zip. Can you link me to the version of MVSPSP you downloaded? If there's a dat file included I can probably take the neogeo.zip I have for FBA and use Romcenter to make it work with MVSPSP.

You need to search for the neogeo Bios yourself. We are not allowed to share it here under the rules of the forum. I am sure you can find it easily using Google

Yeah but he needs the right one. Just googling neogeo.zip could get the latest MAME v0.131 set or a much older version. Check the documentation of MVSPSP if there is any to see what version of MAME it uses. The latest version of MVSPSP I could find any info on through a quick search was 2.3.1, but I couldn't find out what MAME version it uses.

Edit: The documentation is in Japanese so no help there. I do know it uses an old version of MAME, not sure which specific one, but I think I may have found a bios set that will work. Check your PM Ben2dx.