[{key1:"value1",key2:"value2"},{key1:"value2",key2:"value2"},{key1:"value3",key2:"value2"},{key1:"value4",key2:"value2"},{key1:"value5",key2:"value2"}]

db.collection.aggregate([{"$match":{key1:"value1"}},{"$lookup":{"from":"collection","let":{id:"$_id"},"pipeline":[{$match:{$expr:{$ne:["$_id","$$id"]}}},{"$sample":{"size":3}}],"as":"seeAlso"}}])