Ive used a sony ericsson c510 abroad on three.
there is no free internet, calls or texts while abroad.
you can find out about threes charges whilst abroad on the three website.
internet is £1.28 per mb (EU/EFTA area) and text messages cost something like 10 -13 p each.
also it will cost money to receive calls, receiving texts is free.
when i was in the EU area last it was cheaper to send a basic text e-mail than to text, and to receive a reply via text.
As for using the internet, i didnt bother much - only the odd weather forcast. but then again my phone isnt always online.
if your concerned about cost turn your mobile internet off and just use it as and when, and keep a close eye on data use before you go abroad then you will have a rough idea.