Well, you could make them multisite which would allow them all to share a single WordPress installation, but it's a bit harder to set up if its not setup from the start.
When the multisite feature is activated, the original WordPress site can be converted to support a network of sites.
See the WP Codex for creating a Network which explains more about the pros/cons, how it works and how to go about it.
Code:
https://codex.wordpress.org/Create_A_Network
However, as you only have 3 sites. It might not be worth doing and may as well just keep them all separate.
I sometimes manage up to 50 WP sites as have done for years. Some I've had for years, some I flip.
So keeping them separate makes it easier come flip time.
But each to their own.