The answer to this question depends on your expectations in these sports. As you have learned by now specificity is king when it comes to achieving the highest goals. There is a reason he pros specialize in one sport. The good news for you is that the aerobic base that comes from XC skiing will transfer well to the other two sports. There will be little transfer from biking to XC or snow shoe racing.
In terms of an effective strategy: Since all three are winter sports, the more base you can build in biking, running and skiing in the off season, the better. When you add intensity you will need to be careful that you don’t overdo it with intensity in all three sports because there will be carry over fatigue from each to the other. Keep in mind that the intensity training is a supplement to and not a replacement for the base training. If you look through the link I sent you about Marit Bjoergen’s career you will see what a tiny faction of her training was made up of high intensity training.