Flat rate shipping problem

Hi having problems finding the soution for my flat shipping rate.

This is what I’m using 1.25+([qty] * 1.25 – 1.25 * 1) which gives me 1.25 for each item… what do I need to do to get it to add 1.25 after each 3 items ?

Any help would greatly be apprecited!

Regards

Stephen T

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