How could I get where the player's mouse is in the 3d world without the Button1Down Evevnt?

You provided bad detail.

You can only get the mouse of a player in a Local Script.

-- Local Script In StarterPack
local plr = game.Players.LocalPlayer
local mouse = plr:GetMouse()

Make sure the Local Script's in a place that it will work.

Now you can get the CFrame of the mouse anytime you want with mouse.Hit. Example,

-- Local Script In StarterPack
local plr = game.Players.LocalPlayer
local mouse = plr:GetMouse()

wait(5)

local cframe = mouse.Hit

There's also a mouse move event that you can use to always tell where the player's mouse is.

-- Local Script In StarterPack
local plr = game.Players.LocalPlayer
local mouse = plr:GetMouse()

mouse.Move:connect(function()
	local cframe = mouse.Hit
end)

Here's a link to the wiki for more events of the mouse.

Good Luck