I think Vorenus would be third in command.

Legatus Legionis - legion commander (usually a patrician)

Military Tribunes - commander's staff (usually men from noble families, with little military experience)

Praefect (Camp Prefect). Praefectus actually is a very broad category and there were so many different prefects. In the reign of Augustus, he created a Prefect of Rome and had soldiers garrisoned in the city, a first for the city as traditionally, no soldiers were allowed within the Servian walls.

Below him would be the Primus Pilus (or 1st spear, the former rank of Vorenus). Vorenus is also an Evocati, which is a re-enlisted veteran and would be commanding other Evocati or veterans who had completed their mandatory years of service and then re-enlisted. Pompey, it has been rumored, had more than 2,000 Evocati.