[{"productType":"Bike","company":"yamaha","model":"y1"},{"productType":"Bike","company":"bajaj","model":"b1"},{"productType":"Bike","company":"yamaha","model":"y1"},{"productType":"Car","company":"Maruti","model":"m1"},{"productType":"Bike","company":"yamaha","model":"y2"},{"productType":"Car","company":"Suzuki","model":"s1"}]

db.collection.aggregate([{$group:{_id:{productType:"$productType",company:"$company",model:"$model"},count:{$sum:1}}},{$group:{_id:{productType:"$_id.productType",company:"$_id.company"},model:{$push:{k:"$_id.model",v:"$count"}}}},{$group:{_id:"$_id.productType",company:{$push:{k:"$_id.company",v:{$arrayToObject:"$model"}}}}},{$group:{_id:null,product:{$push:{k:"$_id",v:{$arrayToObject:"$company"}}}}},{$replaceRoot:{newRoot:{$arrayToObject:"$product"}}}])