Assuming you are on cingular, the
easiest is simply to call them and activate international roaming...you keep the same number, people can reach you and you will pay $1.29/minute to make and receive calls. If it is truly simply for emergencies and to touch base quickly and you will not be making very many or very lengthy calls, this is the easiest.
You will have to make sure your phone has international capabilities...at the very least you need a phone with either 900 or 1800 frequencies....you'll be okay in the big cities...better is a phone with both 900 and 1800....unfortunately cingular sells tri bands geared to its service and if you've been reading all the stuff, the United States in its usual contrary minded way uses different frequencies than most of the rest of the world...Cingular uses both 850 and 1900 in the USA so their tri band phones only have room for one of the world frequencies which in this case would be 1800 leaving you without 900 service which could be detrimental in many parts of the world but not the big cities in Europe for the most part.
Now next up the ladder, you can buy a cheap European dual band (900/1800) either on ebay, or from internet stores or when you arrive in Europe for around $50 US...the phone has to be unlocked ( if bought on the web it probably is but you have to make sure it is dual band (900/1800)) or you can buy a worldwide tri band (900/1800/1900) which would be a little more expensive.
But if you're going in that direction, the next step for the 3 countries would be to buy an international sim card say from
www.united-mobile.com.
It depends on whether this is a one time only trip or whether you're serious about having mobile phone coverage while traveling in Europe.