How to Put 3 players in one team 4 in the other 6 in the other one?

Well lets do this, We will create a loop that will run for every player and add them to a team

Assuming you have a max number of 13 players,

--variables
local team1 = game.Teams.Team1
local team2 = game.Teams.Team2
local team3 = game.Teams.Team3

--shuffle
local players=game.Players:GetChildren()
local new = {} 
for i = #players, 1, -1 do
    local x = math.random(1, i)
    table.insert(new, players[x])
    table.remove(players, x)
end
players = new

--set
for i,v in pairs(players) do
    if i<=3 then
        v.Team=team1
    elseif i>3 and i<=7 then
        v.Team=team2
    elseif i>7 and i<=13 then
        v.Team=team3
    end
end

Make sure to change team, team2, and team3 to the teams in your game.

Hope this helped if you have any questions please ask me :D