Mercenaries

From EU3 Wiki

Mercenaries may be bought from the recruit management interface. If mercenaries are available they will be displayed at the bottom portion of the window. Mercenary availability will differ with each geographic area and each area has a pool of available mercenaries that may be purchased by any country. If no mercenaries are available in the province you select, other nations beat you to buy the mercenaries available from that pool and you will need to wait for it slowly replenish before you can purchase your own.

While a mercenary unit will immediately appear in the province it is purchased in, it will still start with a low morale rating and will need time to increase it before it can function at full capability. Mercenary units also have some distinct disadvantages in comparison to regularly trained units.

They will contribute only 1/3 of the amount of military tradition that a regular unit would after a battle, will not upgrade when you change your preferred unit type, are twice as much to maintain and will immediately disband should you go bankrupt.