The tests are much more likely to successfully diagnose you if you take them near your peak viral load. Multiple tests over several days gives you more chances to sample near that peak.
If it were just that then taking the tests three times at once would have a similar effect. Instead, it's mostly that you're measuring a quantity that's varying over time, and more samples gives you more chances for one of them to be at a time when the quantity is high.
The math is more intuitive if you say it decreases the failure rate. i.e. if you try something with a 10% success rate 3 times, your odds are 1 - (0.9 ^ 3) = 27.1% chance of success.