Originally Posted by DaveSSS

We know because the vast majority of frames use English BB threads, except for Italian frames and Eddy Merckx frames for many years.

All you really need to do is measure the BB shell width. If it's 68mm, then it's English. Italian shells are 70mm wide.

Originally Posted by storckm

I wouldn't think you'd need a longer chain, unless your triple has larger gears than the double--unlikely, or if you replace the rear derailleur with a longer cage one. You might need a longer or shorter bottom bracket spindle, depending on what cranks you get. Similarly, you might or might not need to replace the derailleurs. Some front derailleurs sold for doubles work fine for triples, in my experience, but I'm sure there are some that wouldn't. Whether or not you need to replace the rear derailleur depends more on the difference between your the number of teeth in your highest and lowest gear. Sometimes, you can get away with not replacing the rear derailleur even though you can't shift into the smallest chainring and the smallest cog at the same time. This is usually O.K., at least for people like me who only use the smallest chainring with the largest three cogs.