Cheapest priced copy (Foundations Jumpstart): $0.39 at Card Kingdom, near mint, their price list as of .
Leonin Vanguard 
Creature — Cat Soldier · 1/1 · Uncommon · Foundations · #499
At the beginning of combat on your turn, if you control three or more creatures, this creature gets +1/+1 until end of turn and you gain 1 life.
Legal inCommanderBrawlStandardPioneerModernLegacyVintage
Not legal inPauper
3 printings · $0.39 to $0.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Foundations | 499 | Uncommon | $0.79 | — |
| Foundations Jumpstart | 216 | Uncommon | $0.39 | — |
| Core Set 2019 | 22 | Uncommon | $0.39 | $0.99 |
2 rulings from Wizards of the Coast
- 13 July 2018If you don't control three or more creatures as Leonin Vanguard's ability resolves, the ability does nothing. However, if you do control three or more creatures as it resolves, the +1/+1 bonus doesn't stop applying if you no longer control three creatures later in the turn.
- 13 July 2018If you don't control three or more creatures as your combat phase begins, Leonin Vanguard's ability won't trigger at all. Causing a permanent to become a creature during the beginning of combat step won't cause Leonin Vanguard's ability to trigger.