It's really only the start of the answer.
For this type of work, budget is king, and second is the desired results. If the desired result is something like what you see on rides at Disney, where a whole wall is filled with perfect video from dozens of projectors, then a budget of $100K or more with weekly tweaking and maintenance is required. If that budget doesn't exist, then expectations need to be reset.
With a three projector setup, you gain a LOT of money back into the pocket, but you lose out on the seamless capabilities, as well as losing perfect color balance between the projectors, but it certainly makes things 'easier' overall.
If you are trying to design this, then I would recommend that you get with a professional A/V firm who can help you with this project and if you intend to use a lot of their time and expertise, then you will need to hire them for engineering purposes. This level of work will require that you know exactly what gear is available and at what price to address the needs of the situation.