One siloed website or three smaller?

I have an idea about an authority site that I want to build and work on as a long running project (indefinitely), but I'm torn between creating three sites targeting different audiences passing some relevant links between them or siloing a single larger site with relevant internal linking.

There are three distinct audiences for the different silos/sites. Two are for professionals (different professionals but the content is in the same niche) and one is for layman/"average joes".

I will be developing the site myself so there are no technical limits in how to structure the site(s).

What would the SEO gurus and IM experts on BHW recommend and why? What's the pros and cons SEO wise of the different approaches?

If you know how to do Siloing properly then it is the way to go.
Read this thread and you'll be ready to fly:
https://www.blackhatworld.com/seo/the-ultimate-guide-to-seo-siloing-dominate-google-in-2018.985967/

I can't say that I know how to silo properly other than on a theoretical level but if it's the way to go then I have to learn to do it proper by going down that route. I'm far more interested in the best way of doing things, not the easy way. Anyone disagree with siloing being the way to go?

If those 3 target groups (your audiences) are close together I'd say go for one site and silo it. The advantage would be that it's easier to get backlinks for a website with 3 times the content. Plus interlinking your 3 sites would never be as effective as getting the same amount of links to just one site.

Lots of good guides on how to Silo properly: http://shaunmarrs.com/silo-wordpress-site/

if the 3 niches you are working on are in the same category, building one siloed site is the best choice since it'll be much easier to build and manage and you'll get better results.

Depends on the size of the niche.
Dont go too narrow