How to display only the products that can be divided by three in the woocommerce shop?

Adam,

Have you tried using the modulus operator? To check divisibility in PHP (and many other programming languages), we have access to the arithmetic operator known as "modulus". The modulus operator ( % ) divides "a" by "b", and returns the result. Thus, all you need to do is check if a divided by b is equal to zero, where "b" is the number you want to divide by (in this case, 3), and if that equates to true, then whatever "a" represented is indeed divisible by "b" (in this case, 3).

Let's take a look at an example:

<?php
$myVar = 9;
$myOtherVar = 10;

$remainder = $myVar % 3;
$otherRemainder = $myOtherVar % 3;

echo "Remainder: " . $remainder;
echo "Other remainder: " . $otherRemainder;

If you ran that, you would see that "Remainder: 0", and "Other remainder: 1". This is just to illustrate how the % ("modulus") operator works. It returns the remainder of "a" divided by "b". So in your code, you should be able to just perform a conditional check on whatever item(s) you want to verify divisibility on, and run the appropriate code should the modulus check return 0.

<?php
$thingToCheck = 15;

if( $thingToCheck % 3 == 0 ) {
    // success, whatever is stored in $thingToCheck is divisible by three, so run whatever code you need to run on those items here!
} else {
    // looks like whatever is stored in $thingToCheck is NOT divisible by three, so run whatever other code you need to for those items
}

Hope this helps!

Erik