Perform joins in mongodb with three collections?

Okay, looks like you've bunch of problems. I'll try to sum them up.

Problem 1 : the aggregate code you've provided doesnt compile.

db.user_movies.aggregate({$match : {mobile_no : mobile_no}},{$unwind: "$movies" },{$lookup: {from: "movies",localField: "movies",foreignField: "movie_id",as: "bmarks"}},{$unwind : "$bmarks"},{$match : {"bmarks.active": 1}},{$group : { _id : "$_id", movies : {$push : "$bmarks"}, movie_ids: {$push : "$bmarks.movie_id"}}},{$lookup: {from: "movie_comments", localField: "",foreignField: "movie_id",as: "comments"}},{$unwind : "$comments"},{$sort: {time_posted: -1}},{$group: {_id: '$_id', comments : {$push : "$comments"}}},

Fix:

db.user_movies.aggregate([{$match : {mobile_no : mobile_no}},{$unwind: "$movies" },{$lookup: {from: "movies",localField: "movies",foreignField: "movie_id",as: "bmarks"}},{$unwind : "$bmarks"},{$match : {"bmarks.active": 1}},{$group : { _id : "$_id", movies : {$push : "$bmarks"}, movie_ids: {$push : "$bmarks.movie_id"}}},{$lookup: {from: "movie_comments", localField: "",foreignField: "movie_id",as: "comments"}},{$unwind : "$comments"},{$sort: {time_posted: -1}},{$group: {_id: '$_id', comments : {$push : "$comments"}}}])

Now assume you're using the same data you have provided.

Problem 2 : {$match : {"bmarks.active": 1}} doesnt match any entry.

Fix : {$match : {"bmarks.active": 0}}

Problem 3 : No look up field defined {$lookup: {from: "movie_comments", localField: "",foreignField: "movie_id",as: "comments"}}

Fix : {$lookup: {from: "movie_comments", localField: "movie_ids",foreignField: "movie_id",as: "comments"}}

Problem 4 : No unwind stage for movie_ids for previous look up

Fix : {$unwind : "$movie_ids"}

Problem 5 : No time posted field {$sort: {time_posted: -1}}

Fix : include field before sorting

So, to put everything together you need aggregate to look like something below for extracting the comments for each movie.

db.user_movies.aggregate([{$match : {mobile_no : mobile_no}},{$unwind: "$movies"},{$lookup: {from: "movies",localField: "movies",foreignField: "movie_id",as: "bmarks"}},{$unwind : "$bmarks"},{$match : {"bmarks.active": 0}},{$group : { _id : "$_id", movies : {$push : "$bmarks"}, movie_ids: {$push : "$bmarks.movie_id"}}},{$unwind : "$movie_ids"},{$lookup: {from: "movie_comments", localField: "movie_ids",foreignField: "movie_id",as: "comments"}},{$unwind : "$comments"},{$group: {_id: '$_id', comments : {$push : "$comments"}}}])

Sample Output

{    "_id": ObjectId("5834ecf7432d92675bde9d83"),    "comments": [{        "_id": ObjectId("583d96d7e35f6e9c53c9e894"),        "movie_id": "dallas00",        "comment": "what a great movie."    }, {        "_id": ObjectId("583d96d7e35f6e9c53c9e895"),        "movie_id": "dallas00",        "comment": "awesome movie."    }]}

Update:

db.user_movies.aggregate([{$match : {mobile_no : mobile_no}},{$unwind: {path: "$movies", includeArrayIndex: "moviePosition"}},{$sort :  {moviePosition:1}},{$lookup: {from: "movies",localField: "movies",foreignField: "movie_id",as: "bmarks"}},{$unwind :"$bmarks"},{$group : {_id : "$_id", movies : {$push : {movie_name:"$bmarks.movie_name", movie_id:"$bmarks.movie_id"} }}},{$unwind : "$movies"},{$lookup: {from: "movie_comments", localField: "movies.movie_id",foreignField: "movie_id",as: "comments"}},{$unwind : "$comments"},{$group:  {_id: "$movies.movie_id", movie_name: {$first:"$movies.movie_name"}, comments : {$push : {comment:"$comments.comment", time_posted:"$comments.time_posted"}}}},{$sort :  {time_posted:-1}},{$project:{_id:0, movie_id:"$_id", movie_name:1, comments: "$comments.comment"}}]).pretty();