If you don't buy membership for 3 months your account/number should still be OK.
It's only if you don't buy membership for 6 months that you may have issues.
See these two parts of the terms & conditions:
Terminating and ending the agreement
Suspending the 48 Services
In particular...
7.3 Where the 48 Services are suspended because you have chosen not to purchase top up credit or make a chargeable use in respect of your Account for a period of 6 months, we will remove any credit on your Account after a further 2 months and credit will not be recoverable. Your service will then remain suspended for a further 5 months and can be reconnected at any time during this period. When the suspension period ends your service will be disconnected, this agreement will end and your number will be immediately lost.
and
8.1 You can end your agreement with us at any stage by not using the 48 Services and choosing not to top up your Account at any stage over a 6 month period
Hope this helps.
