The first comes in Ed Sheeran's verse on "End Game", with the lyric: "Four words on the tip of my tongue, I'll never say it." The logical explanation of the "four words", given the permanency of the rest of the song are: "Will you marry me?"
The second comes on "King of My Heart", when Taylor asks: "Is this the end of all the endings?" – perhaps a reference to her wondering whether her current relationship is "it" for life, and therefore won't have a messy end.
The third reference is in "New Year's Day", with the line "You and me forevermore", which is pretty self-explanatory.
And, finally, some people have questioned whether the line "My name is whatever you decide" in "Don't Blame Me" is a reference to marriage, and the possibility of Taylor changing her name.