I would pay 3x the salary, because a 3x developer creates a lot more value than that
Many people (including op) look at the 3x in a linear fashion. They think the 3x programmer is working 3 times faster, like a 3ghz CPU to 3 1ghz CPUs, or that they work 3 times longer, or a combination of these. This leads to 3 times more code, documentation, delivery etc. of exactly the same quality
I've never met such a programmer
Usually the <magnitude>x comes from better quality, and less quantity. So the 3x programmer has achieved 3x the functionality as requested, but that code base is likely 2x - 3x simpler, with 3x less bugs, which combined is closer to 6-9x easier to maintain. This code also influences the cost of code built on top of it. So the value increase in some areas is exponential rather than linear. You're unlikely to get this exponential increase in value from 1x developers
Another exponential value characteristic of 3x developers is that they have much better decision making. Maybe that means choosing the right tool for the job (choosing a distributed DB in stead of MySQL when you know you need scalability) or designing a customer facing UI. Recovering from a wrong decision can be very costly
These exponential traits mean that the earlier you are in a project, the more value there is in hiring that 3x developer