Covered well by above posters how to address it , Keep it simple, as post # 2 suggested verbiage.
I'll add when I have a "weak" comp, aka one that sold substantially above or below prevailing prices/ days on market/other, I like to use it as a fourth comp, ...if possible. Not all markets offer 4 decent comps but if I can find it, I'll add a comp over a year old ( very easy to make a time adjustment if needed ) or one that sold in competing area so there are 3 solid comps in prevailing terms , with the weak comp[ #4. listing or pending comp plus analysis of listing pending activity frames the picture.
For example, the property sold after only ## days on market, and based on other sales in the area would likely have commanded a higher selling price with more exposure time.
The saying, "show, don't tell" .I'd add a comp to shore up my argument above. The above would likely....with only 2 other sale comps, is shaky, with 3 or more sale comps it's solid. It's important to " tell" in narrative, but narrative is best "shown" by including more facts. ( closed sale prices or listings in pending contract are facts ) It's a harder to challenge facts vs theoretical explanation...
How do you support and explain an adjustment for a comparable
The explain is verbiage others suggested, the support comes from the facts. That said, I would not typically make an adjustment for such a comp. If the low price is so far out of range I'd drop it as a comp. If the low price is relatively reasonable for the DOM, it is what it is, use it as a fourth comp with a brief verbiage why it sold lower. If you can't find other good sold comps to support your theory of would likely have sold for more, it's a weak appraisal.
