hello,
I suggest that you take a look at the binning tool . In the menu at Raster / Geometric / Level-3 Binning
You can use it from the GUI for your first tests but you can use it also from the command line with gpt.
See also the help for more information.
Take also a look at this thread, might be helpful too.
Level 3 binning of Polymer processed OLCI Rrs product - s3tbx - STEP Forum (esa.int)