dashes and commas

I have a grammar question that perhaps someone will know how to resolve. If one has an apposative clause separated by dashes within a series, does one use a comma after the closing dash? Does one use a comma instead of the closing dash? Semi-colons?

For instance: this is the first item--which is modified--, followed by another, and then a third?

Or should it be: this is the first item--which is modified, followed by another, and then a third?

Or something else entirely of which I'm as-of-yet unaware?

Thanks in advance for any grammarian assistance.