>>33342595For Mew you need either a GBA flashcart, or DS flashcart with a DS with GBA slot, and the FireRed savefile used at Toys R Us in '06 to distribute Mystery Mew by direct trade.
Put the savefile and a FR rom on a flashcart, which will allow you to trade a Mew. Alternatively, with a save dump/inject program on a DS flashcart, you can load the Mystery Mew savefile onto a FireRed cart via Slot 2.
For Celebi, not realease outside Japan, so the process is complicated without a Japanese GameCube and the Japanese Colosseum Bonus disc. Easiest way is you need a GBA flashcart, and find a savefile online that has a Japanese Celebi, and trade it to your game.
For Jirachi, you need a GameCube, GCN-GBA link cable, and the Pokemon Colosseum Bonus Disc. Works with Ruby and Sapphire carts, but can then be traded.
For Deoxys, you need a GBA flashcart and the Aurora Ticket distribution rom. Works with FireRed and LeafGreen carts, but can then be traded.