In ranked I have been in many 3v1 situations (on Attack), and I'll usually have one teammate who wants to fight the 1 guy instead of going to the other site to plant the spike.
The conversion rate for a 1v3 is LOW, but it's not so low that it rarely happens. As you can see, even pros lose 3v1s.
Even in Immortal, the players would rather fight the 1 guy instead of planting the spike and playing a 3v1 post plant, which makes it much more difficult for the 1 guy.
A 3v1 post plant where the 1 guy has to come to you, kill all 3 of you, and then defuse the spike, is much more difficult than the 3 of you running at him, while he gets to sit back and play aim labs. Many noobs in Immortal don't understand this concept. Pls and stop throwing ranked games TY!