Q:

If you said  to Allah you will try not to do a sin and does it. What you have to do ? Will it be promise if you said you will try not to do it. If it is a promise, is it Okey if you just fast for three days because you do not want do other things.

A:

You ought to offer expiation (kaffarah) for breaking the oath you have made:

Allah says in the Quran:

“Allah will not take you to task for oaths which you may have uttered without thought, but He will take you to task for oaths which you have sworn in earnest. Thus, the breaking of an oath must be atoned for by feeding ten needy persons with more or less the same food as you are wont to give to your own families,  or by clothing them, or by freeing a human being from bondage; and he who has not the means ( to do so) shall fast for three days [instead]. This shall be the atonement for your oaths whenever you have sworn [and broken them]. But be mindful of your oaths!’ Thus Allah makes clear unto you His messages, so that you might have cause to be grateful.” (Qur’an: 5:89)

Based on this verse, the expiation for breaking an oath entails one of the following:

1-  One should feed ten poor people offering them same food or equivalent in cash or providing them clothes.

2-  The second option of freeing a human being from bondage is not available now as slavery is has been abolished, and we are not allowed to re-introduce it;

3-  Therefore, if a person is unable to feed ten poor persons, because of lack of material means, then he ought to fast three days by way of expiation.

Therefore, if you are not able to offer fidyah of feeding the poor people then you can opt for fasting three days.

Almighty Allah Knows best.