I watched a 2016 UC video about ArcGIS Portal. In the video, they stated that it is recommended that ArcGIS Server and Portal for ArcGIS be installed on separate machines.
They also referenced an optional data store. It sounded like this is where content from analyses are stored. Does this data store need to be installed on a separate, third machine?
Honestly, it kinda of depends on your use case. If you're deploying a DEV instance, expect a minimal number of users, or just want to explore the functionality of ArcGIS Enterprise, the all-in-one ArcGIS Enterprise installer might be your best bet. It'll deploy ArcGIS Server, Portal for ArcGIS, the ArcGIS Data Store, and two ArcGIS Web Adaptors on a single machine.
Portal itself is relatively lightweight - really, it's just a website. ArcGIS Server is where the weight of the processing happens. I'd consider hosting the data store on a separate machine - especially if you're going to work with the Spatiotemporal Data Store (It's recommended that the SpatioTemporal Store be on it's own machine).
Here's a few links to help understand various deployment scenarios. Basically, you can scale as small as one machine, or scale out as required.
All this means is that without context, site and server sizing can be a challenging question to address, and it honestly depends on the use case, expected load, performance and availability requirements.