Pine spacing

Several years ago I planted a grouping of pines in my front yard, it is basically three pines planted in a triangle about 15ft apart. The purpose was to get rid of the lawn in this area and have free mulch every year from the pine straw. But, living here I do have to worry about the diameter of the trees if they someday get blown down. Open grown pines around here can get huge making them dangerous. I have noticed in nature that the closer they are growing together, the smaller diameter the trunk. I have seen them growing as close as 1ft apart.
My question is how close can they be planted together without affecting the health of the trees? Could I plant one or two more in the middle of that triangle? And, would it help slow the diameter growth?

The greater their basal diameter, the less likely they are to blow down (thin trees are weaker!). Keep them at the wide spacing.

Resin

The pines that I planted are pinus palustris which are strong wooded and usually uproot before they break, if they get damaged at all. Pinus taeda on the other hand, usually always breaks mid-stem rather than get uprooted. I guess what I'm getting at is if a slender pine falls on the house it's no big deal. I can pull it off and do some trim work and be done. If a big fat pine falls on the house it's another story (if I'm not in a nursing home by then). It's pretty bad when you have to plan out your disasters. lol The easiest thing to do would be to not plant them at all, but I love longleaf pines and don't have anywhere else to put them that's not dominated by hardwoods.

Pines are good at high winds. People do run into problems if they cut down a bunch of pines leaving a few, basicly they grew with the support of the other trees around them so when you remove the other trees you end up with weak pines. Make sense? As far as what you did I wouldn't plant any more they'll be fine.

Your right about size, I have pine that is at least 3 ft in diameter and 40 ft tall and it's around 20-25 years old. If that falls on something look out. But I've seen it in 50 mph winds and it didn't look like it was going anywhere. In fact in high winds live oaks tend to loose huge branches and I would be more worried about an oak. BTW I've seen live oaks tipped over after a hurricane, roots and all. Their top heavy but I've never really seen a pine do that.

Core, that's exactly what happened at my parents place. They had hundreds of loblolly pines and their yard was basically a pine forest. A man who owns a small sawmill came by and offered to buy some of them. Well, they let him cut down about half of them (I didn't know until afterwards) and after Ivan 90% of what they had left was snapped in half.