How do you plan to return from Tolmin to Bohinj?
For three days it's good to have blister plasters, sunscreen, hat and as light a backpack as possible

Along the Soča the nicest things in my opinion are:
Kozjak waterfall, Zaslap pasture above Drežnica Plains and Polovnik (huge WWI), swimming in Nadiza, Bavšica valley and Zadnje Trente with Zapotok pasture at the head, silence of Na Skali village and Čisti vrh. All more easily reachable. The nicest spots in Posočje are often hidden so it's good to get a bit lost and just explore.
For overnight stays try the hut at Mangart Saddle, at the source of Soča and in Lepena. They are cheap and basically starting points for tours. For example the autumn hike to Mangart Saddle on foot charmed me.
Otherwise everything there is super.