Google's John Mueller replied to a webmaster who is tasked with merging and migrating three different web sites into a single web site. John gave him advice in the Google Webmaster Help thread - the typical advice and encouragement you'd expect from a Googler. But he did tell the webmaster not to worry about duplicate content when doing the migration.
John wrote, "don't worry about duplicate content in the meantime." Here is the full recommendation:
Migrating three sites into a single one is a pretty big step, so I'd recommend taking the time to prepare everything, document all the changes you're planning on making, and monitor them when they're happening. Free2write's link ( https://support.google.com/webmasters/answer/6033049 ) is a good place to start. It gives you information on changes like this in general. Take your time, go at the problem systematically, double-check every step, don't worry about duplicate content in the meantime, and things should go fairly well!
I guess John is saying, do the merge, let things settle and pick up. Then as you see certain pages ranking and you want to adjust that, then maybe merge some duplicate pages afterwards. But first, migrate the sites together and track it before you merge multiple pages into one?
I've seen many cases of merging two sites into one, but three - that is not always that common.
Forum discussion at Google Webmaster Help.