Is it necessary to make up for the three days concerning which there is doubt about breaking the fast due to bloody discharge preceding menstruation, especially since there is a fourth day for which breaking the fast is certain, knowing that the questioner is pregnant and fears exhaustion from making up the days?

You are required to make up for the day you broke your fast due to menstruation. As for the three days during which you observed discharge accompanied by a small amount of blood and did not break your fast, your fasting on those days is valid. What you saw is not considered menstruation. The default is that fasting is valid, and it is only invalidated by the occurrence of something that definitively breaks it.