-  An uncle

The given sentence has a subject-verb agreement error, an error in the use of a preposition, and a mistake in the use of tense.

• "The uncle of mine" is grammatically incorrect. It should be "My uncle" or "The uncle of mine."
• "Who is farmer" should be "who is a farmer" because "a" is used to specify that the uncle is a farmer, and "who is farmer" is not grammatically correct.
• "Gave me a piece of useful advice" is correct as it is.
• "When I went to see him three week ago" has a tense error. It should be "when I went to see him three weeks ago" because "ago" is used to indicate a time in the past, and "week" should be pluralized to "weeks."

In conclusion, the corrected sentence is: "My uncle, who is a farmer, gave me a piece of useful advice when I went to see him three weeks ago."