d3.csv("file0.csv").then((data0) => {
    console.log(data0);
    d3.csv("file1.csv").then((data1) => {
        console.log(data1);
            d3.csv("file2.csv").then((data2) => {
                console.log(data2);
            })
    })
}).catch(function(error) {
     // handle any error that may occur from three calls.
});