How can the fast of two consecutive Ramadans (2015 and 2016) be made up, and what is the ruling on making up the fast of Ramadan 2017, which is likely to be broken due to pregnancy, and is expiation obligatory in these cases?

There is no sin upon one who delays making up for missed fasts due to an excuse. If one delays making up for missed fasts from multiple Ramadans, they should make them up whenever it becomes easy for them. However, it is not permissible to delay making up for missed fasts until after the subsequent Ramadan when one has the ability to make them up earlier. Expiation (kaffarah), which is feeding a poor person for each day, becomes obligatory for delaying without a valid excuse. As for delaying with a valid excuse, nothing is required of them.