Hi Jony
The only way to expand tonal range is use more exposures. I've written a lot about HDR and I'll be honest and say that the +/- 2 or 3 EV bracketing is rubbish.
I've got an HDR hints, tips and FAQs tutorial
here but in essence you need lots more exposures.
My usual range is -5 to +5 in 1 EV stops so thats -5, -4, -3, -2, -1, 0, +1, -+2, +3, +4 and +5 (11 exposures). If you think each shot is capturing a particular dynamic and tonal range then having -3, 0 and +3 leaves "gaps" where there could be more data. In reality what this means is that you don't get as good a tonal gradient.