Once the defender has placed all their hidden set up markers the attacker then deploys their forces as described in the Scenario Set-up rules. Once the attacker's deployment is complete the defender reveals their hidden set up markers and places the appropriate models on the table. For a unit of troops or multiple vehicles one model is placed on the counter with the rest of the unit in 2" coherency as normal. A lone vehicle must be placed on the counter itself. Counters which are for units in reserve have a chance of being a minefield or a booby trap.
Revealing Hidden Troops
Source: 3rd-A-Palooza (3.5) Rulebook v1.4, p. 60