This has been discussed many times, networks CAN detect if you are using your handset as a modem, and while I have used me Handset & XSeries for the odd, sneaky bit of browsing, I wouldn't make a habit of it because as the previous poster states, IT'S IN BREACH OF YOUR TERMS & CONDITIONS!
In answer to your original question, yes, while you may be able to access the internet using internet max or even XSeries - 3 are albe to charge you whatever they wish for data as you'll be in blatant breach of said T & C's.
If you want to do more than the minimum of browsing using a laptop, then activate 'Add Broadband Lite'. It's only a fiver more than you currently spend for 'add internet max', you can use it on handset or with PC, and you wont be breaking the rules and leaving yourself open for a hefty bill.