The key thing is to talk to your close friends directly. Just tell them you're planning something for the summer and ask if your date might interfere with their plans. Often people are way more flexible than we think, and if they're really your friends, they'll understand. Plus, it might turn out that a wedding the day or week after theirs won't bother them at all - distances work out, they can afford it, or they're actually happy to have another reason to celebrate.
If three weddings in July-August feel like a critical conflict, look at the gaps between them. Maybe there's a week or week and a half between someone's wedding and the next one? That's where you could fit yours in. Fall is also a good option, but not because you're avoiding conflicts - it's more that you might just like it better there: the weather, the décor, the vibe. Switch to fall if you actually prefer it, not because you're scared the venue will be empty.
And honestly, not that many people will turn down guests just because there are two weddings on the same day. They'll probably split between the events or pick whichever one's closer to them. The main thing is give people a chance to plan their lives instead of hitting them with a done deal.