Company merger - impossible 3-months rewrite

It was a fun time, our company acquired two other companies and we were told that we have 3 months to integrate all the products into a single, unified platform. So you would sign in and could use one, two, or three products in any configuration seamlessly. We estimated the effort for around 1 year of work and we got 3 months to deliver it. Short version - we managed to do so. How we did it is a sum of many different things - mostly having very engaged members within our team. This is how it went down:

I think the important part here was that I and the technical cofounder knew the app very well and we were able to spot potential roadblocks (at least most of them) and have a rather clear vision of how the whole thing would work from top to bottom and we were able to document that vision clearly as JIRA tasks.