score:1

something like this will work :

d3.csv('linktocsvfilehere.csv',function(data){

var sumarray = [];
var janarray= [], febarray= [], marcharray = [];

data.foreach(function(d){
   janarray.push(d.jan); //push all jan values into jan array
   febarray.push(d.feb); //push all feb values into feb array
   marcharray.push(d.march); //push all march values into march array
});

var jansum = getsumofarray(janarray); //get sum of jan array
var febsum = getsumofarray(febarray);
var marchsum = getsumofarray(marcharray);

sumarray.push(jansum,febsum,marchsum);

console.log(sumarray); //here is your array you want

function getsumofarray(array){

   var thissum = 0;

   for(var i = 0; i<array.length; i++){
      thissum += array[i];
   }
   return thissum;
}
})

score:2

even though thatoneguy's answer is perfect, this is way shorter:

d3.csv("yourfile.csv", function(data){

    var totalsum = [d3.sum(data.map(function(d){ return d.jan})),
       d3.sum(data.map(function(d){ return d.feb})),
       d3.sum(data.map(function(d){ return d.march}))];

    console.log(totalsum);//[4, 9, 12]

});

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