You can either drive on the A3 towards Chur and take exit Unterterzen, park your car at the cable car station Unterterzen and take a cable car up to Tannenboden. This would be the recommended
option, as parking up in Flumserberg Tannenboden is limited and it can get very full on weekends.
Or you can continue on the A3 towards Chur and take exit Flums, drive up to Flumserberg Tannenboden itself and park there. Parking is limited, esp. on weekends!
Please note that you need to pay for the parking spots.