db={events:[{_id:"1",title:"foo"}],confirmations:[{_id:"1",eventId:"1",userId:"1"},{_id:"2",eventId:"1",userId:"2"}],users:[{_id:"1",name:"X"},{_id:"2",name:"Y"}]}

db.events.aggregate([{"$match":{"_id":"1"}},{"$lookup":{"from":"confirmations","as":"confirmations","let":{"eventId":"$_id"},"pipeline":[{"$match":{"$expr":{"$eq":["$eventId","$$eventId"]}}},{"$lookup":{"from":"users","as":"user","let":{"userId":"$confirmations.userId"},"pipeline":[{"$match":{"$expr":{"$eq":["$_id","$$userId"]}}},]},},]}}])