Amillenial Christians do not believe in a literal 1,000-year reign of Christ. They believe Christ is now sitting on the throne of David and that this present church age is the kingdom over which he reigns. Although I could not find any specific reference to how they view the Festival of Trumpets, the Day of Atonement and the Feast of Tabernacles (or Booths), I did find an article that explains how they interpret unfulfilled prophecy as being “spiritual” or non-literal:
The amillennial view comes from using one method of interpretation for unfulfilled prophecy and another method for non-prophetic Scripture and fulfilled prophecy. Non-prophetic Scripture and fulfilled prophecy are interpreted literally or normally. But, according to the amillennialist, unfulfilled prophecy is to be interpreted spiritually, or non-literally. Those who hold to amillennialism believe that a “spiritual” reading of unfulfilled prophecy is the normal reading of the texts. This is called using a dual hermeneutic. (Hermeneutics is the study of the principles of interpretation.) The amillennialist assumes that most, or all, unfulfilled prophecy is written in symbolic, figurative, spiritual language. Therefore, the amillennialist will assign different meanings to those parts of Scripture instead of the normal, contextual meanings of those words. Source: https://www.gotquestions.org/amillennialism.html
It would seem that amillennial Christians do not anticipate any literal fulfilment by Christ of the three autumn/fall festivals.
Preterism falls into two camps: full (or consistent) and partial. Full preterism takes an extreme view that all prophecy in the Bible has been fulfilled in one way or another. Partial preterists take a more moderate approach, and many partial preterists consider full preterists to be guilty of heresy. The following extracts sum up partial and full preterism:
According to partial preterism, there is no rapture, and passages describing the tribulation and the Antichrist are actually referring to the destruction of Jerusalem in AD 70 and the Roman emperor Titus. Partial preterists do believe in the return of Christ to earth and a future resurrection and judgment, but they do not teach a millennial kingdom or that Israel as a nation has a place in God’s future plan. According to partial preterists, the Bible’s references to “the last days” are speaking of the last days of the Old Jewish Covenant, not the last days of the earth itself.
The partial preterist viewpoint leads to a belief in amillenialism (or post-millenialism) and is associated with covenant theology. Of course, it rejects dispensationalism. But its main problem is its inconsistent hermeneutic and its allegorizing of many biblical prophecies that are better understood literally. While partial preterism is within the scope of orthodoxy, it is not the majority view among Christians today. Source: https://www.gotquestions.org/partial-preterism.html
According to preterism, all prophecy in the Bible is really history. The preterist interpretation of Scripture regards the book of Revelation as a symbolic picture of first-century conflicts, not a description of what will occur in the end times. Preterism is the view that the biblical prophecies concerning the “end times” have already been fulfilled—in the past.
Preterism is the view that all the end-times prophecies of the New Testament were fulfilled in AD 70 when the Romans attacked and destroyed Jerusalem. Preterism teaches that every event normally associated with the end times—Christ’s second coming, the tribulation, the resurrection of the dead, the final judgment—has already happened. With regard to the final judgment, it is still in the process of being fulfilled. Jesus’ return to earth was a “spiritual” return, not a physical one. Source: https://www.gotquestions.org/preterist.html
I can only conclude that since amillennial Christians have no expectation of the Rapture of the Church or that Christ will return to earth and dwell with His people, any reference to Jesus fulfilling the last three fall feasts are to be taken spiritually and not literally. Preterists believe end-time prophecies have already been fulfilled, and the final judgment is in the process of being fulfilled.
Edit: From the book "Revelation: Four Views" the Foreword says this about amillennianists:
...the amillennianist believes that the Bible does not predict a period of universal peace and righteousness before the end of the world. Instead, good and evil will coexist until the second coming of Christ when the dead are raised and the last judgment held." (p xiii)
The book presents examples of how those in the four main 'schools' of Revelation interpretation uphold their various stances. Yet within the four main groups, there are many variations, and some cross-over. A lot is not clear-cut. So, within amillennianists there are differences of views on various points even if they are all agreed on the definition above.