this is a combined metro area of well over 2 million people.
of the three you mentioned, harrisburg would be better than lancaster, and both would be far better than york.
again, the suburban sprawl around this area is enormous, so you wouldnt necessarily have to live "in" either city to take advantage of the singles scene.
I am reaching out today to absorb friends into my life. About Guys I Want To Meet: All I ask is that you are who you are. Workout daily, would love a workout partner, for spotting purposes only.
also, wilmington, DE is about an hour from baltimore and would offer more than harrisburg, but is still largely "suburban" in character. My advice would be to reconsider Baltimore or either Philly. It just depends on what you are use to I guess, but south central PA is way behind the times in a lot of ways.
it also has the added benefit of being about 25 miles from philadelphia, and is close to other neat towns, like west chester, PA. There is also an abundance of obese and unhealthy looking people from what I saw.
The shop-worn advice is to find folks with some kind of shared interest - you would come on too strong if the shared interest is just a sham, but if you have enough of a shared interest then being single doesn't matter as much to the shared interest.
The Harrisburg area has enough outsiders coming through, in different age categories, so if clubbing isn't your thing, there are other groups accommodating shared interests. One example is the hiking club - The Online Susquehanna Appalachian Trail Club - which happens to be where I met my wonderful wife!