If you want to be able to activate it using Wifi , so without spending money using the 3g data connection, you will have to make a custom doctor.
One of the options you can set, there, is to do the first time activation using wifi.
So basically... download the stock doctor for your model ( Eu or at&t ), look at the thread here,
WebOS Internals Meta-Doctor
and make a meta doctor with the options of your liking, then doctor your pre3 and activate it...
You will want to wait to put the simcard inside IF it's a brand new phone, in case you want to associate it with a specific app catalog... it's an operation that can be done just 1 time, at the beginning. If you put the sim, you cannot do it anymore.