Yay! Nerd question!
OK, Pennsylvania, according to the 2017 numbers, had a population of 12,805,537 (2017 Census estimate). The United States as a whole has a population of 325,719,178.
So, if we assume that the members here are randomly distributed across the US, the chances of any single trade partner being from Pennsylvania is 3.93% (about 1 in 25.4), and the odds of all three being from Pennsylvania would be 3.93% x 3.93% x 3.93% = 0.006%, or about 1 in 16456.
But wait, we also have members from Canada. Not sure how many or what percentage, but that means we need to add in the 35,151,728 Canadians, So that means that Pennsylvania has 12.8 million out of the combined population of 360,870,906, and the odds of a single trader being from PA is 3.55% (1 in 28.2) and the chances of all 3 is 0.0045% or 1 in 22380.
Of course, all of this assumes that the members here are evenly distributed across the US and Canada and it ignores the possibility of members from other countries, so without having the actual demographics of the membership, it's all guesswork, but I'd suggest that 1 in 20,000-25,000 seems like a reasonable guesstimate.