margin shorthand with 3 values is merged into one

Input:

.klt-p-block-heading {
    margin: var(--klt-l-spacing-1, 0) 0 var(--klt-l-spacing-3, 0);
    position: relative;
}

Expected output:

.klt-p-block-heading {
    margin: 40px 0 30px;
    position: relative;
}

Given output:

.klt-p-block-heading {
    margin: 40px;
    position: relative;
}

Works fine when using margin with 4 values and two values. But not with 3 values. Possible to happen with other shorthands too.

Reproduce the three-value margin case shown in the issue, then locate the plugin entry point and existing tests for CSS variable transformation. Compare the handling of two-, three-, and four-value shorthands and add regression coverage for the three-value case. Done means the example preserves all three resolved values without regressing the other shorthand forms.