Well, I pretty much just dump a packet of seeds in a little dug up space next to the trellis and they seem to do fine. Honestly, except for a radish accident I never bother to thin my annual seeds.

I'm sure this isn't the technically 'correct' answer. :-/

I think it rather depends what you are going for. If you are looking for the best blooms for cutting you want good strong plants, and they need rich soil, training and plenty of space. If you just want a pretty climber they could be closer but personally I think they need more than 3 inches. To keep them blooming they need rigorous dead heading. You could thin them and transplant the thinnnings. You don't have to sacrifice any plants.

Thanks for the advice.