I think the earlier Model 3's had no gap between the bottom of the rear window and the boot lid opening so water on the back window would trickle directly into an opened boot.
Later models have a channel between the glass and the boot opening for the water to flow into.
A few months ago I ordered a rain spoiler for my, still to be collected, M3 from AliExpress.
Kind of fortunately it got lost in the post and I've been refunded.
When I took a test drive in an M3 LR a few weeks ago I noticed this new rain channel so spoiler not needed now.