How can i make it print child number 3's name?

Since GetChildren returns an array/table you can just do this.

print(game.Workspace:GetChildren()[3].Name)

You could do: (I don't know why you would do this but ok)

local t = workspace:GetChildren()

local c = 0
for i,v in pairs(t) do
    if c == 3 then
        print(v.Name)
        c = c + 1
    else
        if c < 3 then 
            c = c + 1
        end
    end
end

If you want to find the child with name number 3 do:

if workspace:FindFirstChild("3") then
    print(workspace:FindFirstChild("3").Name)
end

If I didn't understand what you meant please leave a comment.

Just a simple way to do it

local ThirdChild = game.Workspace:GetChildren()[3] --Gets the third value from the Array