Because pH is a logarithm, its steps do not add — they multiply. One pH unit down is TEN times the hydrogen ions; three units down is $10 \times 10 \times 10 = 1000$ times. So “how much more acidic?” is answered by counting steps and stacking tens: pH 3 against pH 6 is three steps — a factor of a thousand, never “three more”.

The same trick runs on the basic side, counting pOH steps in $[\mathrm{OH^-}]$. This count-the-steps habit is the exam's cheapest point and the working chemist's favourite estimate: land the power of ten FIRST, and any calculator work afterwards merely confirms what you already knew.