What exactly do you mean by "have been swapped to Three" (I presume you mean that you unilaterally decided to move to Three - and port your number from 48?) and "at the same terms"?
If you decide to move again from Three back to 48 then you'll be a new user with all that that entails.
E.g. you'll need to create a new account, get a new SIM, pay the once-off activation fee of €12.99, buy the €12.99 monthly membership plan (unless you qualify for the €10.99 student offer) etc.
If you need to contact 48 support about this then see the link in my signature below or email them at support@48months.zendesk.com
