Twin-pack kit 2-528 from the Association for four plank (O3) and five-plank (O5) wagons for ca 1890-1950.
The information from the 2mm Association website suggests using the 2-361 chassis for both O3 and O5. This is correct, but not very helpful. In particular, only about 200 four-plank wagons were built to diagram O5, using the 9 ft DC underframe, whilst over 20,000 were built using the earlier conventional single-sided brake system without a diagram number, of which about 18,000 survived to be updated to double-sided version with the diagram number O21 in 1927-1930 (both of which are represented by underframe 2-330 and 2-335 conversion kit for steel solebars, together with separate axleboxes and springs).
It is fairly trivial to create a three-plank wagon from the four-planks O5, simply by filing off one plank before assembling the body (fortunately there is no diagonal strapping on four-plank and less).
Components
Five plank (O5), and Four plank O3
Optionally
Four plank other than O3
Literature
A History of GWR Goods Wagons by Atkins, Beard, Hyde & Touret (David & Charles)