Ok, I don't belive noone did this yet, so I will. Here is the Quote from the book
Check this: first two lines say must you ride and must you light. But third says will you know. Also in in first 2 lines are words one, and in third are once. Why is this? I'm sure George was thinking how to form this propercy very hard, and there is reason why this last line is a bit different. What do you think it is?
Here is what I think:
It literally says she will ride 3 mounts and that she will light 3 fires. It is something she will do. But it also says that she will know 3 treasons. It, acually dosen't say someone will betray her. It could be that she will betray someone (when she got her Unussulied - it is a betrayal), or (very little likley) someone will betray someone else, and that betrayal will teach her something.