Get only 2or3 documents from every collectiongroup Firestore

I am trying to get 3 documents from every collectiongroup
issue: only 3 documents getting of first collectiongroup name. please help?
My code is following:

let ids=[1,2,4,5,7]
firebase.default.firestore().collectionGroup('posts').where('user_id', 'in', ids).orderBy('timestamp','desc').limit(3).get().then(data=>{
console.log("result:", data)
})

Data structure in Firestore:

users (collection)>>1 (doc)>>posts (sub-collection)>> 100 of docs (in data “user_id”=1)
users (collection)>>2 (doc)>>posts (sub-collection)>> 100 of docs (in data “user_id”=2)
users (collection)>>3 (doc)>>posts (sub-collection)>> 100 of docs (in data “user_id”=3)
users (collection)>>4 (doc)>>posts (sub-collection)>> 100 of docs (in data “user_id”=4)
users (collection)>>5 (doc)>>posts (sub-collection)>> 100 of docs (in data “user_id”=5)
users (collection)>>6 (doc)>>posts (sub-collection)>> 100 of docs (in data “user_id”=6)
users (collection)>>7 (doc)>>posts (sub-collection)>> 100 of docs (in data “user_id”=7)
so on…