I think that when it is hosted in a single country is better but usually FIFA has done so when the host country has some other countries near it that love football also,I remember in 2002 this was the case and the World Cup was held in Japan and South Korea and it had a lot of contention because of the referees favoring the host countries in a clamorous way,Italy lost 3-0 to South Korea something which was unimaginable until then.That is why I am more in favor when a single country is hosting the World Cup rather than holding it with other partners which can bring us to this non grateful situations where big teams are out of the play because of referees favoring home countries,it is much better when it is just one home country.
I agree about letting one country host the world cup instead of three countries but in fact, since we have more counties playing in the 2026 world cup maybe that's why FIFA let three countries host the world cup however, there won't be three hosts for the world cup, the country hosting the world cup in America and two other countries are just helping America.
I think it is important to remember that three nations mean a lot more things nations could offer while hosting, that is why it's being done right now. Saudi Arabia is promising a lot of stuff as a single nation for 2030 for example, but it would be like Qatar where one nation will spend so much money and time and workforce to make it happen, all these Ronaldo transfers and maybe potentially Mourinho and Messi, all those to attract people to Saudi league and make them accept 2030 as a potential.
However, would you rather have a single nation that is not ready and will get ready, or would you rather have 3 nations that is ready today? That seems like a better idea which is why I am not against it too much, this one has too much distance however, that's an issue, like two nations bordering easily could be fine, but a game at New York and a game at Mexico has so much distance.