When resubmitting a paper, should I write individual letters for each reviewer or one common letter?

Usually, you sent back one single letter in which you answer the reviewers questions one by one. I always start with a short note to the editor, listing the major revisions done as bullet points. I also thank the reviewers for their time spent on the review and that their comments advanced my manuscript (of course I write this only if the review really improved my work). Then I start with "Reviewer 1" and list each of his questions or remarks and mark my comments either in bold or in a different color.

I think that @Jens already provided the right answer, that usually you answer all the reviewers in a single response. In my experience, all the ACM and IEEE journals I've been involved with as author/reviewer/editor share the response letter with the reviewers, who then see each other's comments and the authors' response to them.

You describe a somewhat unusual situation here, in which the editor has more major comments than the individual reviewers. Even then, I think I would send a single response, and assume the reviewers are going to see the response to the previous submission, including the editor's concerns. If you have reason to think otherwise, asking the editor the question you have posed here seems appropriate.

By the way, in my experience usually the letter from the journal says specifically that the authors are to respond to the comments and explain how they have addressed each concern.