Currently the unique 2-handed mace Skywarden is bugged and you need it for the exploit to work.
If you don't know what it does, the text on the weapon reads: "Every 60 seconds, gain a random Law for 60 seconds".
First, pick your favourite law and equip it with whatever rune.
Next, equip the Skywarden.
Wait 60 seconds for the unique ability to activate and another law should should come on.
Now unequip the Skywarden.
Normally the 2nd law would disappear if you did this, but it won't and will persist till forever as far as i know. Even if you change the first law you picked to another skill.
At this point you can equip your preferred weapon and run around with 2 laws active or...
You can re-equip the skywarden and wait another 60 seconds.
What will happen is the 3rd unused law will become active and you should now have all 3 up at the same time.
In order for the 3rd law to remain active. you have to keep the Skywarden equiped, also zoning in to a new area will remove the 3rd law but it will come back after 60 seconds.
If you unequip the Skywarden at this point the 3rd law will disappear, but the 2nd will not.
Edit:
I like to use Law of Valor as my main law.
After some playing around I've found that if the 2nd law that activates is Law of Hope, it will not persist after unequiping Skywarden.
If Law of Justice activates then it WILL persist and the rest of the exploit works as I've explained.