Database
db={"Users":[{"key":0},{"key":12}],"Posts":[{"key2":2},{"key2":2},{"key2":2},{"key2":2}],"Chatrooms":[{"key3":2}]}
Query
db.Users.aggregate([{$count:"TotalUsers"},{$lookup:{from:"Posts",pipeline:[{$count:"TotalPosts"},],as:"posts"}},{$lookup:{from:"Chatrooms",pipeline:[{$count:"TotalChatrooms"},],as:"Chatrooms"}},{$project:{TotalUsers:1,TotalChatrooms:{$arrayElemAt:["$Chatrooms.TotalChatrooms",0]},TotalPosts:{$arrayElemAt:["$posts.TotalPosts",0]}}}])