[{x:0},{x:1},{x:3},{x:4},{x:5},{x:6}]

db.collection.aggregate([{"$facet":{"smaller":[{"$match":{x:{$lt:3}}},{$sort:{x:-1}},{"$limit":1}],"larger":[{"$match":{x:{$gte:3}}}]}},/** cosmetics to revert back to original form*/{"$project":{final:{"$setUnion":["$smaller","$larger"]}}},{"$unwind":"$final"},{"$replaceRoot":{"newRoot":"$final"}}])