Play at least three Knights
The first sole leader with at least three played Knights qualifies for Largest Army. The award contributes two points while held. A Knight still in your hand contributes nothing to the played-Knight count.
Other development cards do not help this count either. Road Building may gain territory and a hidden victory-point card may improve your score, but neither is an extra soldier toward Largest Army.
Allow one card play per turn
You can play at most one action development card in a turn, and a newly bought action card is not usable that same turn. You cannot buy three Knights and reveal all of them to claim the award immediately.
That makes early preparation valuable. A player with two played Knights and an older unplayed card may be much closer to the award than someone holding a large fresh hand of development cards. You can see the played count, not their hidden card identities.
Overtake the holder rather than matching them
If the holder has three played Knights, another player reaching three does not take the award. The holder retains a tie. Reaching four while the holder stays at three creates a new sole leader and transfers the two points.
Reassess the race after each Knight. Continuing to buy cards solely for the award becomes less attractive when another player already has a large public lead and your own economy needs buildings.
Use the Knight’s immediate benefit too
A Knight also moves the robber, which can free your own important tile or block a productive opponent. If an eligible opponent has cards beside the destination, it can lead to a random theft. Playing the card does not trigger the seven-roll discard rule.
Choose the moment for both benefits: immediate production relief and progress toward the award. Treating every Knight as only one-third of two points misses much of its value to a resource-starved economy.