3GW counts separately local (domestic) and roaming usage. However the handling of roaming is still limited by comparison with local usage (no billing rules).
The roaming usage graph and status text in the main screen roaming section are available only if you define a roaming quota (in Settings > Roaming).
Widgets can show actual, left, or predicted roaming usage (the two last ones only if there is a roaming quota).
You can choose to count roaming usage as local (i.e. to count the two types of usage together). This can be handy in countries like Spain where in some plans all usage is roaming.