If the transmission has never been touched and the vehicle has high mileage,probably hard miles, the trans maybe developing a lot of end float on the main shaft assembly , allowing excess movement ,resulting in the noise in 3rd.3rd gear is at the front of the main shaft and as such is likely to move around more than say 1st or 2nd.It may also have been run low of oil resulting in a burnt gear set.
Seems like you need to pay for some diagnostic work. They can help you out.
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