I've heard people about people who get the master sword with three hearts but the thing is that I can not find any more shrines so far and I have 12 hearts!
Is there a pretty simple way to get he master sword with 12 hearts?
There is no way around the 13 heart restriction to get the Master Sword; unless you hack the game that is. If you wish to gain the Master Sword, you have two options:
Based on this guide, those who have the Master Sword during 3-heart challenge runs take advantage of the The Statue's Bargain quest. They will do shrines until they reach enough stamina that is equivalent to 10 hearts. They will temporarily exchange their stamina for hearts, get the Master Sword, and then go back to 3 hearts
There is no intended way for you to get the master sword with less than 13 hearts. You have essentially 3 options (2 of which @Wondercricket already mentioned in his answer):
TL;DR: Just look for 4 more shrines, or use an interactive map.
There is also a glitch you can use with the horned statue in the quest mentioned above that can duplicate heart containers/stamina vessels.