Sounds good to me and technically following the Selling Guide.Quote Selling Guide B4-1.3-08 "The subject property can be used as a fourth comparable sale or as supporting data if it was previously closed."
Put as comp #4 or put as 1-3. If put as 1-3 say, comps arranged by gross adjustments required. State subject is used as a 4th comparable sale.
Use it as addl support but if you can't support your conclusion with other sales then have you supported value?