How to I Architect my application?

So I have been tasked with a project where I have 3 entities, say a retail shop, wholesale shop and a manufacturer. Each have their own schemas (each shop has its own subdomain). All three entities have requested for their own database but they want frequent transactions happening between the shop, wholesaler and manufacturer.

I have been thinking about this for a while but could not think of a good viable approach. What would be the right approach to execute?

Can you elaborate on the kind of transactions you want between them? What about building 3 separate apps, separate databases, and then defining your own internal API to communicate between them for those transactions?