As a RAM_STORE storage mode, what would be the ideal amount of RAM to perform routing using the API at an average of 3 req/s on the North American continent?
Take a look at this blog post: Host Your Own Worldwide Route Calculator With GraphHopper - GraphHopper Directions API
Generally you want to fit the graph (*-gh or graph-cache) folder in RAM plus some additional memory for the actual routing (not that much if you use CH for example).