with eye of zurd when you have 12 cruelty
you may reroll one of your dice for every 4 cruelty you have
so you may reroll 3 of your dice at once or you may still reroll one or two dice?
That is correct - you can reroll any one dice up to 3 times during a conquer action (depending on the amount of cruelty you have)
i mean its mandatory to reroll 3 dice if you choose to use the effect of eye of zurd?
Nope - the keyword in the Eye of Zurd text is "may" which means that it is your choice whether or not you want to use the card ability (and how many times you want to use it) during a conquer action.