So they range together, they just sleep separately? If that's the case, they're already getting used to each other. I say a few weeks before you decide is the absolute cutoff for having everyone in the coop, put two groups together, let them all adjust for a little bit, then add the last group when you have to. They already see each other and are familiar, so the usual let-them-get-used-to-each-other-through-the-fence period is already in play. Expect some pecking and rank establishing, but as long as it doesn't draw blood, it'll be fine, and is completely normal.
Anyone else have thoughts to the contrary?