Some hints from the Fannie Mae Selling Guide, B4-1 (if your subject is rural):
"A minimum of three closed comparables must be reported in the sales comparison approach."
"Additionally, older comparable sales that are the best indicator of value for the subject property can be used if appropriate. For example, if the subject property is located in a rural area that has minimal sales activity, the appraiser may not be able to locate 3 truly comparable sales that sold in the last 12 months. In this case, the appraiser may use older comparable sales as long as he or she explains why they are being used."
"Rural properties often have large lot sizes, and rural locations can be relatively undeveloped. Therefore, there may be a shortage (or absence) of recent truly comparable sales in the immediate vicinity of a subject property that is in a rural location. Comparable sales located a considerable distance from the subject property can be used if they represent the best indicator of value for the subject property. In such cases, the appraiser must use his or her knowledge of the area and apply good judgment in selecting comparable sales that are the best indicators of value. The appraisal must include an explanation of why the particular comparables were selected."
Only two of the three sales needs to be manufactured, also.
"A minimum of three closed comparables must be reported in the sales comparison approach."
"Additionally, older comparable sales that are the best indicator of value for the subject property can be used if appropriate. For example, if the subject property is located in a rural area that has minimal sales activity, the appraiser may not be able to locate 3 truly comparable sales that sold in the last 12 months. In this case, the appraiser may use older comparable sales as long as he or she explains why they are being used."
"Rural properties often have large lot sizes, and rural locations can be relatively undeveloped. Therefore, there may be a shortage (or absence) of recent truly comparable sales in the immediate vicinity of a subject property that is in a rural location. Comparable sales located a considerable distance from the subject property can be used if they represent the best indicator of value for the subject property. In such cases, the appraiser must use his or her knowledge of the area and apply good judgment in selecting comparable sales that are the best indicators of value. The appraisal must include an explanation of why the particular comparables were selected."
Only two of the three sales needs to be manufactured, also.