All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Since marriage is of this status, it should not be undermined by uttering words of divorce for trivial reasons, especially since the wise Sharee’ah did not make divorce the first solution for a troubled marriage. In addition to this, it should be noted that both spouses are ordered to treat each other kindly and each one should respect the rights and status of the other: Allaah says (what means): {And due to them [i.e. the wives] is similar to what is expected of them, according to what is reasonable. But the men [i.e. husbands] have a degree over them [in responsibility and authority]. And Allaah is Exalted in Might and Wise.} [Quran 2:228]
On the other hand, anger does not prevent divorce from taking place unless the husband is not conscious of what he says as we explained in Fatwa 121374.
It is based on these details [in the above referred Fatwa] that the ruling on divorce is issued, i.e. whether or not it had taken place when you uttered it while in a state of anger; as you are more aware of your own situation.
Three divorces pronounced in one session take place as three divorces according to the view of the majority of scholars and this is the view we adopt here in Islamweb. However, Shaykh Ibn Taymiyyah and his student Ibn Al-Qayyim consider that three divorces in one session take place as one divorce. For more benefit, please refer to Fatwa questions 200411 and 89861.
We do not know on what basis you took your wife back the first time after having uttered divorce three times. However, if you consider it to be the most evident scholarly opinion because you are a student of knowledge, or because you followed the opinion of a scholar whose knowledge you trust and he issued you a Fatwa that three divorces in one session take place as one divorce, then in this case, you are not sinful.
Please note that the expression that your wife wrote: “Divorce happened” does not result in anything as divorce is in the hands of the husband. Also, the words of the husband to his wife: “Consider yourself divorced” is a metaphor of divorce and it does not result in divorce except with intention. In addition to this, a divorce in writing is also a metaphor of divorce as clarified in Fatwa questions 87034 and 89894.
As for divorcing a pregnant wife, it is not against the Sunnah, so the fact that the wife is pregnant does not prevent divorce from taking place.
Finally, since the matters which we have mentioned require finding out from you your intention when uttering divorce expressions and your state of anger, then in our view, it is better to consult an Islamic Center in your country or verbally ask a trustworthy scholar there.
Allaah Knows best.