db={"productCollection":[{"_id":1,"name":"Laptop Dell 22inch","details":2}],"detailsCollection":[{"_id":2,"brand":7}],"brandCollection":[{"_id":7,"name":"Dell"}]}

db.productCollection.aggregate([{$lookup:{from:"detailsCollection",localField:"details",foreignField:"_id",as:"brand"}},{$lookup:{from:"brandCollection",localField:"brand.brand",foreignField:"_id",as:"brand"}},{$addFields:{details:"$$REMOVE",brand:{$arrayElemAt:["$brand.name",0]}}}])