(1) I am finished with that customer.
(2) I have finished with that customer.
(3) I finished with that customer.
I understand (2) and (3) but (1) seems strange to me.
Could you explain to me please:
What's the difference in meaning between (1) and (2)?
In what situations should we use (1) and in what (2)?
What's the difference in meaning between (1) and (3)?
In what situations should we use (1) and in what (3)?
loviii(1) I am finished with that customer.
First, you need to determine the meaning of #1. It depends on the context.
1. Levi has a long line of customers waiting to be served.
Wen he concludes the business with the customer he is dealing with, he can say: "I am finished with this customer; the next one in line can come in."
2. Harry is a customer. Harry is very demanding. He is not nice to Levi. He is never satisfied and wants more.
After Levi concludes his business with Harry, he can say "I am finished with that customer. I never want to see him again. He is the worst customer ever, and I refuse to talk to him again."
In a waiting line context, #1 is the usual and very common expression. #2 is understandable and ok, but not preferred, and I personally would never use #3.