They're all correct and there's no difference in meaning. As long as you follow the general "subject-verb-object" word order, modifiers can be placed almost anywhere.

I would say that 1. "Sometimes these things happen" 2. "These things happen sometimes" Are both used, however, I would say that "Sometimes, these things happen" is used 9 times out of 10. Which one you use depends on the context of the sentence before it and how you're delivering it.