The 3rd Squad provides another march slot once you meet its unlock conditions, including Base level 20. It lets you keep more assignments running without recalling a squad that is already busy.
For example, you can leave gathering squads at resource tiles and use another available march for a Radar task. The benefit comes from having a free slot as well as the heroes and units to fill it.
How deployment works
A squad is occupied while deployed. Its heroes cannot be changed during the march, and it must return before being sent on a different assignment. When every available squad is out, further deployments need a squad to return or another slot to be unlocked.
The 1st, 2nd and 3rd Squads are separate marches. Raising one building's level does not create another march.
Building upgrades and research
The 3rd Squad building adds power and a march-speed bonus as it levels up. Squad 3 research covers other improvements, including combat stats, gathering and Load Capacity. Load Capacity concerns carried resources; Hero Squad Size concerns units in the formation. Neither should be confused with the number of march slots you have unlocked.
For the difference between deployment slots, unit capacity and resource load, read Squads Explained.
