How to do i make it every time i click a part it randomly generated a number out of 3?

The reason this doesn't work is because the random number was already chosen before the function. So your num is before the function so that means its randomized and then every time your function runs its always one number because the random number was before the function. To fix this, just put the local variable INSIDE the function, so each time the function runs, a random number is chosen, not the script you have which already has a chosen number before everything, and it stays like that forever.

Fixed Code:

-- move the local num inside the function
    local BrickEvent = game.ReplicatedStorage.Events.Gui

    BrickEvent.OnClientEvent:Connect(function(plr)
local num = math.random(1,3)
-- random number inside function so that every time function runs there's random number
    if num == 1 then
              print("you got Critical Hit Fact")
            script.Parent.ScreenGui.Frame.HintCriticalDamage.Visible=true

     elseif num == 2 then
             print("you got Stamina Fact")
            script.Parent.ScreenGui.Frame.HintStamina.Visible=true
     elseif num == 3 then
             print("you got Jump Attack Fact")
            script.Parent.ScreenGui.Frame.HintJump.Visible=true

            wait(12)
            script.Parent.ScreenGui.Frame.HintCriticalDamage.Visible=false
            script.Parent.ScreenGui.Frame.HintStamina.Visible=false
            script.Parent.ScreenGui.Frame.HintJump.Visible=false
            end
        end)