I suspect this is the case. There aren't a huge number of triple sockets on the market as it is and those that there are seem to be priced more as a niche item than a volume product.but maybe they did not perceive a large enough market to justify the effort..
You do see "combination plates" with more than 2 13A sockets and no fuses but they seem to reuse double socket mechanisms with separate terminals for each pair of sockets.