How can we choose the third kits of teams like manchester united in the pc version of the game. when i play a friendly, i can only choose the home kit or the away kit?

U can't, have to unlock it first. I think.

how do you unlock fifa 2004 kits like the third kits

You have to beat them to unlock each of their kits.

i have already started a career mode and beaten nearly all the premier league english teams. I still dont have any kits.

Go and check in your profile U must have got them.

i got all the premiership kits in hall of kits as i beat them all in practice mode.