I've discovered, from another forum, that Irfanview will open Merrill XF3 files. You need to install the plug-ins with the main programme.
It seems to be opening a proxy, or inbuilt preview, rather than the raw file, it opens the file pretty quickly. The colour is not quite correct and high iso files look grainy, but it has a Thumbnails view and a simultaneous Full Frame view, so it is useful for browsing XF3's if SPP is acting up. Irfanview is free.