In other words, the North Country, despite it's crushing defeat, has not given up completely on their invasion.
If it is assumed that the current commander will be let go because of his complete defeat, who will his successor be?
How do they intend to supplement their diminished troops?
Will they just break the recently agreed upon truce without an honorable justification?
Various factors are entangled over those three years.
At a minimum it would take three years.
So, in reality it will take longer.
"But that's good. In three years time Shirone should be able to rebuild itself."