There was no tercio concept as a field formation. There was a bastioned square (just one way to arrange the "mangas" around the "escaudron"), and other formations.
Imperial troops sometimes fought in deeper formations than, say, Dutch regulations required, and sometimes in larger units as well, but much of what has been written about this comes from English language writers with an axe to grind, that the protestant powers had a more modern and flexible formation approach. This is why we are also told that Imperial cavalry caracoled whereas protestant cavalry charged. (Caracole wasn't really even a formation or battlefield behavior, it was just a way to manoevre/change formation.) The fact is that Imperial infantry were fighting in fairly long lines with the musketeers on the flanks well before the TYW started.
Breitenfeld is given as the ultimate example of Imperial infantry fighting in deep clumsy formations, but even if you believe Horn's account, that was not doctrine, it may have been dictated by circumstance on the field that day. But Horn (a swedish general) is the only writer of the period to claim the few huge units. All other period accounts claim the usual number of Imperial infantry regiments -- 10 to 14.