How to evolve my onyx,scyther,and slowbro by trading?

You can evolve Onix and Scyther by leting them hold the item Metal coat and trading them.Slowbro doesn't evolve.Slowpoke is the one that evolves to either Slowbro at Level 37 or to Slowking When you let it hold a King's Rock and trading it.You can trade with a friend who has gameboy and either Pokemon Gold,Silver or Crystal.You both need a gameboy not a gameboy advance and you need one gameboy link cable.Then you both link each others gameboy's and turn on both games.You then both go to any Pokemon center,go to the top floor and talk to the first lady from the left.She says welcome to CABLE CLUB TRADE CENTER.......and then asks you if you want trade,both of you say yes and I think you save after that.Then you trade the Pokemon with the item I said for the pokemon.It will evolve in your friends game if you put the item I said.Then you tell your friend to trade it back,Make Sure He/She Trades It Back.There you have your new evolved pokemon.

Attach a link cable to another gameboy with the same game and they will evolve, if it's your friend they will trade you back right away. Also some Pokemon need to be holding certain items

Well, you had part of it right, so I will help you out with the rest. When you said how can I trade my three Pokemon to get them to evolve into their next form, you were right about that. However, only two of those Pokemon evolve by trading. In order to get your Scyther to evolve into a Scizor, you will need to trade him to someone while he is holding the item called Metal Coat. In order to get your Onix to evolve into a Steelix, you will also have to trade him to another game while he is holding the Metal Coat. Unfortunately, there is only one Metal Coat that can be found in the game. You will have to choose one of the Pokemon.

Hope this helps.

acutully theres 2 one in iron island and one in the same spot when u finish the game cuz i have

Actually, you can CHEAT!!!!!!!!!! ;) but I think they only sale it at target