Must one make up the prayers missed during the menstruation period, the bleeding of which stopped on the third day, knowing that the bleeding was very light, and then you discovered that you are pregnant? And how can one easily make up 15 prayers?

The correct view is that a pregnant woman does not menstruate, and any blood she sees is considered istihadah (non-menstrual bleeding). She must make up the prayers she missed due to this blood, according to her ability and provided it does not harm her body or livelihood.