Depends on which country you go to. If it is a 'feel at home' country it won't cost any more to use abroad than it does at home,
USA, Italy, Republic of Ireland, Austria, Australia, Sweden, Denmark, Sri Lanka, Hong Kong, Indonesia, Macau.
From 1 July 2014 you’ll also be able to enjoy Feel At Home in France, Switzerland, Norway, Finland and Israel.
Any other country and it will be standard roaming rates and the exact cost will depend on whether you are on contract or pay as you go
http://support.three.co.uk/SRVS/CGI-BIN/WEBISAPI.DLL?Command=New,Kb=Mobile,Ts=Mobile,T=International_charges,varset_cat=roaming,varset_subcat=4157,Case=obj(5013)