I'm happy with this game but the dragon stones, zenies and treasures drop is so scarce. Is there any way to farm faster?
For dragon stones, it appears that they are dropped mainly while doing the main story quests. Sometimes there are rewards on events, and sometimes on log ins.
Story quest give a stone for each level on each difficulty and a 2nd for completing a story group on each difficulty. Events depend on the event, usually 1 stone for finishing all difficulties on strikes or 1 per difficulty for story events with several stages like the gohan event. Then you have several different missions that give one or more stones. Also some of the login bonuses give stones.
Other than logins, once you do all these things you can't get any more free stones until more content is added. So no repeated farming.
Treasures and Zenies
Concerning treasures and zennies, it seems that
Area 16 – Reunited: Stage 8 – Ending of the Body Exchange is a great place to farm a boss called Pilaf, who drops 10 Gems and several thousands of zennies.
A complete tutorial and precise article is describing everything you need to know here: Dokkan Blog. The author is explaining how he farmed all those gems. He also precises: running a stage on higher difficulty level grants better drop rates.
It is best to run this on Z-Hard, as the drop rates are obviously higher when the difficulty is higher[...].