When you approach your multisport training, the best way to answer your questions is to better understand the principles behind the work you are putting in to improve. 

These are basic principles of sport training you will want to keep in mind:

Individuality
Everyone is different and responds differently to training. Some people are able to handle higher volumes of training while others may respond better to higher intensities. This is based on a combination of factors like genetic ability, predominance of muscle fiber types, other factors in your life, chronological or athletic age, and mental state.

Specificity
Improving your ability in a sport is very specific. If you want to be a great pitcher, running laps will help your overall conditioning but won’t develop your skills at throwing or the power and muscular endurance required to throw a fastball fifty times in a game. Swimming will help improve your aerobic endurance but won’t develop tissue resiliency and muscular endurance for your running legs.

Progression
To reach the roof of your ability, you have to climb the first flight of stairs before you can exit the 20th floor and stare out over the landscape. You can view this from both a technical skills standpoint as well as from an effort/distance standpoint. In order to swim the 500 freestyle, you need to be able to maintain your body position and breathing pattern well enough to complete the distance. In order to swim the 500 freestyle, you also need to build your muscular endurance well enough to repeat the necessary motions enough times to finish.

Overload
To increase strength and endurance, you need to add new resistance or time/intensity to your efforts. This principle works in concert with progression. To run a 10-kilometer race, athletes need to build up distance over repeated sessions in a reasonable manner in order to improve muscle adaptation as well as improve soft tissue strength/resiliency. Any demanding exercise attempted too soon risks injury. The same principle holds true for strength and power exercises.

Adaptation
Over time the body becomes accustomed to exercising at a given level. This adaptation results in improved efficiency, less effort and less muscle breakdown at that level. That is why the first time you ran two miles you were sore after, but now it’s just a warm up for your main workout. This is why you need to change the stimulus via higher intensity or longer duration in order to continue improvements. The same holds true for adapting to lesser amounts of exercise.

The principles of specificity, progression, overload, adaptation, and reversibility are why practicing frequently and consistently are so important if you want to improve your performance. Missed sessions cannot really be made up within the context of a single season. They are lost opportunities for improvement. Skipping your long ride on weekend A means you can’t or shouldn’t go as far as originally planned on weekend B (progression & overload). Skipping your Monday swim means your swimming skills and muscles won’t be honed or stressed that day (specificity). Missing a week due to a vacation sets you back more than one week (adaptation and reversibility). Apply these principles to your training to get a better understanding of your body and how to achieve success.