you need 3 separate shipping rules:
up to 3 nappies:
triggers:
Subtotal for any column;
if the subtotal of column 'quantity' is '>' a value of '0'
AND
Subtotal for any column;
if the subtotal of column 'quantity' is '<=' a value of '3'
Calculation:
Flat rate charge of '7.20'
4 to 19 nappies:
triggers:
Subtotal for any column;
if the subtotal of column 'quantity' is '>=' a value of '4'
AND
Subtotal for any column;
if the subtotal of column 'quantity' is '<=' a value of '19'
Calculation:
Flat rate charge of '11.40'
20 + nappies:
triggers:
Subtotal for any column;
if the subtotal of column 'quantity' is '>=' a value of '20'
Calculation:
Flat rate charge of '14.50'