Underline vs Hyphen vs Space in URL

Does it make a difference?

[drshieldsatjeemail]

Is there a major difference between the following url naming technique for SEO?

mysite.com/three-key-words.htm

mysite.com/three_key_words.htm

mysite.com/three20%key20%words.htm

It seems that I am seeing more and more sites using hypenated url filenames.

Thanks in advance for any input.

[Stefan]

Welcome to WW, drshields.

I don't think underscore versus hyphen makes much difference (except that perhaps underscore looks less like SEO to SE's these days - but that's pure speculation), but the %20, I don't know... it might be best to avoid that version.