Like Bill O mentioned, it isn't that three storm systems are “converging” on the summits nor are their three tracks converging on the summit at the same time (usually). I think a lot of people picture the movie/book “the Perfect Storm” in which they made it appear that the storms were all converging at once against impossible odds. This is partially true but Lows merge and combine often (look at 12-12-10 for a good example at that).
A storms track can play into the strength of the winds we get but the concept of the three storm tracks (not systems) explains the frequency of the high winds we get on the summit. On average there are three common directions that storms come from and they all typically follow water ways of sorts. The first is the Great Lakes lows that travel up the St Lawrence River. The second are lows from the Ohio River Valley that usually form in the Gulf or Mexico region and the third being up the Atlantic coast line. There are several other directions that they come from but these on average are the three most common ones. A good paper to look at is a paper by Van Cleef published in the 1908 Monthly Weather Review (citation number 36:56-58 as well as Chart XII on page XXXVI-38 and Chart XIII on page XXXVI-39). This will help explain storm tracks across the US. There are other ones to review and a simple google search or library search will find them but this is the paper behind the map we show tourists up here on tours. I would recommend a physical copy for a clearer picture but you can read it online here: http://books.google.com/books?id=zjh...201908&f=false. It is the frequency of passing lows that has earned New England the nickname “the Tailpipe of the US” because most lows that form somewhere in the lower 48 will usually make their way towards us (see the averaged low map to see what I mean). And it is this frequency of passing lows that allows the summit (on average) in the “winter” (Oct to May) to see a hurricane force gust (73+ mph) every other day and a century gust (100+ mph) one out of every four days.
There are several factors that go into the winds we (and others) get: a storms track and proximity, the pressure gradient between incoming and exiting highs/lows, a lows/highs vertical stacking at various levels in the atmosphere, wind direction at each level, sheering or veering, jet streaking, temperature gradients horizontally and vertically, the air density horizontally and vertically, how a low fills-in, how a low deepens, friction on the surface, color of the terrain and vegetation, type of terrain (water vs land), the elevation of the terrain and the shape of the terrain (Brad touched upon both of these a bit), as well as location of measuring it all. I’m not sure of your meteorological background so the three to focus on for us are pressure gradients, the shape of the land, and the height of the land. Those are the ones that we use on tours and are the easiest for most people to “get”.
Hope this helps get you steered in the right direction and if anything else, feel free to ask and I will try to respond timely. If all else fails, a private message works to get my attention since it is linked to my email.
Staff Meteorologist/Night Observer, KMWN (Mt Washington Obs., NH)