Even when adopting Project Management, in the end all is done by people.
Technology changes and has to be adapted to the situation.
Processes may change and SHOULD change. Moreover, I like to see processes as necessary bureaucracy, to be reduced to the minimum; they are a “necessary illness”.
A firm needs products, not processes. Products are delivered by people making use of the right technology.

As for the company size and the number of people, take the military example: provided one does not overdo with bureaucracy and processes, a military unit works well whatever its dimension, be it a platoon or a corps.
A military unit must keep bureaucracy and processes to a minimum, needs to be technologically up-to-date, can’t work at all without the properly trained people.

Therefore, I would add a fourth element: training.
You need a serious and good Project Management methodology, honest people and … good training.