In order for the skill to trigger with an ally unit, the team must have fewer than 3 allies sharing a movement type on the field.
Ostia's Pulse takes account of ally units on the field at the start of the phase, meaning if allies are lost it is possible to meet Ostia's Pulse's condition and trigger the skill.
Example: A team consists of 1 armored unit with Ostia's Pulse & 3 Fliers. The skill cannot trigger with the 3 Fliers because there are more than 2 of them on the field. Should the third flier be defeated, Ostia's Pulse's condition will be met and the skill will trigger.
Infantry, 
Armored, 
