Although it isn't clearly specified and needs to be brought up in Q&A, I'd tend to agree that there will be three trailers per alliance on the field regardless of the number of robots, both to avoid providing a competitive advantage to teams that fail to bring a robot and to simplify field reset mechanics.
Based on past observation of match procedures, I expect Field Reset will probably reposition the trailers in their launch pads and reset the orbit balls to their proper starting locations while match N-1 is removing their robots. Then they'll flag the match N teams to bring in their robots, hitch up, and adjust position within the launch pad. This avoids having to carry the trailers in and out of the arena, which would have two negative effects:
- Increasing turnover time by adding to the things being carried through the field entry/exit gates
- Requiring designating a trailer parking lot in the queuing area, which is already quite congested in some venues