I am not sure this writer knows what he is talking about. My Dad used to service some equipment for the Secret Service garage where these are maintained and stored. At the time there were way more than 3 presidential limos. I remember my Dad saying they were numbered and had little markers that they put on top with the number and they randomly drew numbers to see which limo would be used for each mission. This resulted in making harder for anyone to potentially sabotage a vehicle.
Additionally, they have to have a whole fleet because the advance teams load up multiples for every place the president travels. On any given trip, they may need 2-3 in multiple cities on a multi-state or multi-country tour so “one of three” does not seem right.
Same reason when the presidential helicopter takes off from the White House, the other two meet it and disperse in different directions to keep people guessing which one the president is on. My son had the opportunity to be at the WH where he got to shake Trump’s hand and watch him and Melania leave on the helicopter. I still have the video he took from that day.