jailbreaking/unlocking iphone3g

hello all, new here have quick question.. bought 3g phone , unlocking t mobile.. downloaded redsn0w right firmware (4.2.1) , go jailbreak it, when hold buttons down near end of last 1 5 seconds left (let go of power , hold home. ) hear sounds device re syncing computer , messes whole process up. (also have tried jailbreak green poison , same thing.. why im stumped...)

 

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