The 72-hour SL pass is sold by Pressbyran convenience stores, the Arlanda tourist office and SL offices. I’m not sure if you can make the 72 hour pass work for you given your 3.5 day visit, but the 72 hour pass period starts from first swipe use on the SL bus/train/tram/ferry.

The 72 hour SL pass is not good for the trains from Arlanda airport, but you seem to know something about that already.

There is an SL office on the lower retail level at Stockholm Central station, the final destination of the Arlanda Express, but I would instead use one of the Pressbyren stores at Stockholm C to buy the 72-hour SL passes. The Pressbyren stores at Arlanda airport also sell the SL passes, but the tourist office at Arlanda is usually more conveniently located for me.