db={A:[{"_id":"asd123","loc":"blr","date":"2020-01-01","test":{"a":true,"b":true,"c":true}}],B:[{"_id":"asd999","loc":"blr","date":"2020-01-01","test":{"a":[1,3],"b":[2],"c":[]}}],C:[{"_id":1,"url":"https://stackoverflow.com/","name":"Rahul","age":24},{"_id":2,"url":"https://stackoverflow.com/","name":"sita","age":26}]}

db.A.aggregate([{$lookup:{from:"B",let:{loc:"$loc"},pipeline:[{$match:{$expr:{$eq:["$loc","$$loc"]}}},{"$lookup":{"from":"C","localField":"test.a","foreignField":"_id","as":"test.a"}},{"$lookup":{"from":"C","localField":"test.b","foreignField":"_id","as":"test.b"}},{"$lookup":{"from":"C","localField":"test.c","foreignField":"_id","as":"test.c"}}],as:"testlinks"}}])