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.
There should be no problem getting Canada's visa if the holder is already holding a US visa as that will be a clear sign that the person holding it is not on anybody else's blocklist. While Mexico is somewhat a free country where you can visit without having any visa for a good 6 months or 180 days to be exact. Moreover, if you're someone who's living in a first world country, you wouldn't have to think of these problems as almost everything can be achievable with just a passport.