Thai law says you can require upto 2 months of deposit, but a renter will ask for 3 months in advance and the 3rd is payment for the first month. Some renters only ask one month deposit in an older home with furniture of little value.
I am surprised that they would request this info for a non resident of Thailand. I think it might be that you listed the wrong country of residence. I always list my residence as the USA, as I am here on tourist visa and my place of residence is the USA and will always be even I have little intent to live there in the future.