As far as I know, the only devices that save photos in Apple ProRAW format are certain iPhones. (ProRAW format is a variant of Adobe's .DNG (digital negative) format that lets iPhones record some computational photography data.)
If you are referring to Raw files in the generic sense,
Pixelmator Support – Working with RAW images
"Note: Pixelmator Pro relies on macOS for RAW support, so the RAW files you can edit in macOS Photos can be edited using Pixelmator Pro."
Digital camera RAW formats supported by iOS 26, iPadOS 26, macOS Tahoe 26, and visionOS 26 - Apple Support
I don't see the Olympus OM-3 in that list – and nobody here can answer the question of when Apple will add OM-3 support to the macOS Raw support libraries. Because Adobe's Raw conversion code supports OM-3 Raw files, one option might be to use Adobe's Digital Negative Converter to create .DNG files from the OM-3 Raw files … and then import those .DNG files into Pixelmator.
Adobe – Supported cameras
Adobe – Adobe Digital Negative Converter
You might need to play around with the output settings in the DNG converter to discover which is the latest version of the .DNG format that Pixelmator will accept.