There is no single right ratio. It depends on your troop count and stats. Here is the reasoning, then a simulator to find your own optimum.

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Open the Troop Ratio Simulator| Ratio (Inf:Lan:Mks) | Notes |
|---|---|
| 10:30:60 | The classic. Marksman-favored but keeps inf/lancer. Default pick. |
| 10:10:80 | Heavier marksman, for strong marksman buffs. |
| 1:1:8 | Almost all marksman with 1 each of inf/lancer. |
| 0:0:100 | Looks strong but tests lower. Not recommended. |
Diminishing returns: troop count scales sub-linearly, so all-in on one type plateaus. Zero-troop penalty: tests show damage drops when infantry/lancer are zero. So marksman-favored but not all-in wins.
The rally leader needs at least one of each troop type to trigger all 3 heroes' skills. Joiners are freer to lean marksman.
The biggest reason there's no single correct ratio is that lethality balance between troop types shifts by generation. The overseas community notes that after Generation 8, marksman lethality far exceeds lancer. As a guide, once marksman lethality is 358+ higher than lancer in Gen8+, cut lancers to 10-15% and push marksman above 85%.
Before Gen8, lancer and marksman lethality are close, so a balanced ~10:30:60 keeping some lancers works well. The right move is to adjust based on your server's generation and the per-type lethality shown in your profile.

Compare sending 180k total as "0:0:100 (marksman only)" vs "10:30:60." Marksman-only looks like max damage, but (1) concentrating in one type slows the gain, and (2) the zero-infantry/lancer penalty applies. As a result, a moderately spread 10:30:60 often gives higher total damage. Japanese strategy site Altema states the same: 100% archers loses roughly 20% versus the 60% ratio, and their base recommendation is also 10:30:60.
This is a "tendency that fits most people"; your exact optimum depends on your lethality stats. Advanced players fine-tune in 1-5% steps to find their peak. The Troop Ratio Simulator compares multiple ratios with your numbers at once.