The 20cwt 3" is not an impossible build. The sighting arrangement is a little complex but simplified it's easy enough to knock one up with plasticard, tubing and some bits from the spares box. You could mount one on a J- Type Thorneycroft from Sloppy Jalopy or do what I did and convert a Lledo diecast. There's a Military Modelling Annual from 1978 that has a great series of drawings. IIRC the Lndships site now has some too
If you want a good way to cheat on the barrel and get a very good approximation I'd suggest you get a Minfigs 4.7" naval gun from their 15mm Colonial range, it scales out nicely for a 3" in 28mm. The 25mm version is also useful for other projects.
The easier alternative is to buy a suitable 18pdr or 13 pdr and mount the barrel on a simple pedastal and fit to teh back of a truck for the 13/18pdr AA gun.
Eureka also do a 3 pdr naval gun. During the war the RNAS had a number of tenders fitted with this gun, they served as an AA battery in and around London and the south coast.