It'd have to be really steep. Regenerative braking on a model 3 is pretty aggressive. You don't even use the brakes in normal driving. If you're starting off too cold that can be an issue though. I've driven in big mountains (Canada, BC) and while you're definitely using a lot more power on your way up you make up for it on the other side. I have worried about not making it to the top though (but it worked out fine for my trips).
Had a recent talk with a colleague who owns one of earlier model S on exactly this. He often goes skiing up to Verbier in the Swiss alps, so similar situation and cold.
He definitely burns through the battery much quicker in mountains, and going down it recoups just a small fraction of energy spent going up. Also cold messes up the numbers pretty significantly, I think he mentioned 10-15% of battery consumed standing still each full day in freezing temperatures (he told me battery keeps itself warm).
Pushing up 1,5 tonnes (or 2+ in tesla's case) of our ICE car can burn up any fuel very fast in relatively short distance. I've estimated that our current petrol and previous (otherwise very efficient) diesel burn around 1/4 to 1/3 of the fuel tank on each 1000m of altitude gained.
