Why do you have to visit 3 countries?, I think if you intend to watch the world cup matches live because your country's national team will play, just look at the group phase where your country will be based camp, after that if they can advance to the next round then the United States is the choice because this country has a larger portion of organizing matches than the other two countries, but if you don't have the funds for that then it's actually not a problem because matches will always be broadcast live and that's enough in my opinion because I will like that too.
That is not a fair argument. There are millions of football fans around the world, who want to watch the world cup matches in person rather than on television. And conducting the tournament in three countries instead of one complicates things for them. The last time world cup was co-hosted was in 2002, when the matches were staged in Japan and South Korea. Even then, the distance between venues were manageable. But that is not the case with USA and Canada. Boston to San Francisco is close to 5,000 km and even by air it will take 6-7 hours. And then there are visa requirements to travel in the US. Getting a US visa can be very difficult.
I confess that I don't have an idea how the average fan goes about visiting tournaments like the World Cup. Are they applying for one game and then go there and spend some time in the country or city? Probably yes because can you even apply for multiple games? If you do, is it likely that you win several ticket lotteries and how do you know which one you are going to win?
When a World Cup game is played in California and a fan from Europe wants to attend that game, a long-haul flight has to be booked to in order to watch that game assuming that the fan won the ticket lottery beforehand.
What exactly is the problem now with three countries co-hosting a World Cup? If a fan from Mexico wants to watch the final and the final takes place in the US, the fan has to travel to the US the same way a fan from Europe has to travel to the US. What is the difference in the planning procedure for any fan from any place when they decide to watch a game?
Visa could be one reason, but I think that the US won't make it unnecessarily hard to get one. If a fan from Washington wants to watch a game in California, that is a hell of a ride. Except for the visa procedure, traveling from Washington to California is at least as exhausting as traveling from Mexico to California unless you are from the deepest South of Mexico.