[{fieldC:{fieldX1:{N:2},fieldX2:{N:1},fieldX3:{N:3}}},{fieldC:{fieldX1:{N:0},fieldX2:{N:0},fieldX3:{N:0}}}]

db.collection.find({"$where":"function() { for (var field in this.fieldC) { if (this.fieldC[field].N > 0) return true; } return false; }"})