After traveling to Europe yearly for over the last 8-9 years I have found certain items just fulfill their function perfectly, so here's my list:
1) ExOfficio underwear - I have found that these are the perfect travel clothing item that does exactly what their intended for, last for years, and are comfortable, easy to sink wash and dries in 4 hours.
2) The small Rick Steves red toiletry bag - This bag has been with me to 8 different countries and is the perfect size for a man, I like the built in hanger for small bathrooms, it just works for me.
3) Travel Multi-outlet - I bought this off amazon and it is a small four plug outlet. I only have to have one adapter for the main plug and it gives me four regular american outlets for my iPhone and iPad. Since my wife and I both carry iPhones and mini iPads it is nice to be able to recharge all four at the same time from one outlet, also I use it to recharge my camera each evening after a day of sightseeing. (All our electronics are 110/220, dual voltage.)
I would be interested in hearing which 3 items you consider essential.