Is takfeer understood as follows:

1. Someone who believe it is unlawful to do something, and yet does it, then he/she has committed a sin. In this it is counted as a sin and would be punished.

2. Someone who believes it is unlawful, but did qiyas/ijtihad and came to the conclusion that it is okay under a particular condition - they may be punished but at the same time rewarded for doing ijtihad.

3. Someone who believes something unlawful to be lawful and does it - then they have disbelieved in the message and are out of the fold of Islam.

If yes, those sisters who openly express that they don't believe that wearing hijab is fardh and that is why they are not wearing it, are they out of the fold of Islam?

This issue you are referring to is known as Istihlaal (having the belief that something that Allah has made unlawful is lawful). There are two types of Istihlaal: that which is related to belief, and that which is related to action.

1. Istihlaal that is related to belief: In this a person believes something that is unlawful in Islam to be lawful, whether he commits the act or not. This is considered to be kufr in all circumstances, regardless of whether this Istihlaal was in relation to something that only reaches the level of minor kufr or something reaching the level of major kufr. For example, if one made fornication to be lawful, or drinking, etc. then he is a disbeliever. Likewise, if he made reviling the Messenger (ﷺ) or reviling the deen to be lawful, then regardless of whether he did the act or not, he becomes a disbeliever by this Istihlaal.
2. Istihlaal that is related to action: This merely means that a person commits an unlawful act and makes it lawful for himself in terms of his action only. In such a situation one must look at the action he committed. Is it something that does not reach the level of major kufr? Such as fornication, stealing, gambling, etc. In this case the person is not a disbeliever by this Istihlaal of action. However, if it is an act (or statement) that expels someone from the deen, then they would leave the fold of Islam.

So based on the above categorization, if these sisters fall in the first category mentioned above, then they are disbelievers. But if they fall in the second category, then they are not disbelievers.

And Allah knows best.