Your first two sentences are grammatically poor English. I look forward IN doing business with you I'm looking forward IN doing business with you There are subtle differences in the 3 sentences. The first one shows your assertiveness and positivity in doing business. This is usually used in written correspondence. This is because you are not using contractions. The other two are conversational because of the contractions used. I'm looking forward in doing business with you. This implies that the deal is sealed and you two are going to enjoy a future relationship together. I'm looking forward to do business with you. This implies that the deal has not been finalized and hope to have a future business relationship. These are the subtleties in which candidates will become more successful. This is especially so in interviews where you are being judged not only the way you speak, the way you present yourself and how you make others perceived as the sort of person they want to hire you.

The first two are both OK.

I'm looking forward to doing business with you.

1...O 2...O 3...X

The first and the second examples are right, but the second one sounds better cause it emphasizes your current urge.