These should really be the sum of all of the verts, not just he first three... Maybe something like

var verts = _.map(this.verts, function(vert) { return object.verts[vert]; });
var x = _.reduce(verts, function(sum, vert) { return sum + vert[0]; }, 0);
var y = _.reduce(verts, function(sum, vert) { return sum + vert[1]; }, 0);
var z = _.reduce(verts, function(sum, vert) { return sum + vert[2]; }, 0);