They're actually quite different when for example you say that you will go London for three days you mean that your trip there will take three days but when you say that you will go to London in three days you mean that you will go there in one , two or three days in other words you are not mentioning the duration of the trip but the time that you will start the trip and that it wont take more than three days for you to start the trip . Hope you got it .