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I'm going by memory here, so bare with me.
To get Gen 3 to Gen 4, you need a regular DS so you can put the cart into the bottom of the DS, then Diamond/Pearl/Platinum into the top part.
Then you need to get far enough into D/P/Plat to access the Pal Park, which allows transfer of Pokemon from Gen 3 to Gen 4.
Then, and this is where I am hazy, but I think you need two DS. One needs to be loaded with D/P/Plat, then the other needs to be loaded with a Gen 5 game (BW, BW2). You need to progress to late/post game in BW/BW2 to access the Transfer Lab or something like that, which unlocks transfer from Gen 4. THEN, you need to put the tentacruel in a box, switch to download play, activate the transfer lab in Gen 5 on the other DS, and it should transfer the pokemon.
Next, to get from Gen 5 to Gen 6, you need Poke Transporter from the Nintendo App store on your 3DS that has Alpha Sapphire. You also need Pokemon Bank, which costs five dollars a year.
Using Poke Transporter, you can transfer your Gen 5 pokemon to Gen 6 games, and from there you are done.
