Is it wise to merge blog-sites?

I have 3 EMD's micro niche sites in a very similar topic. None of them is doing good now after all the updates.
I am considering to merge them to one site. So there will be more content on one site.

Is it wise?
Wouldnt i get duplicate content penalty, or something similar?

If i proceed, should i point the old sites to the new/chosen one or just drop them?

Thank you for your opinions.