Is it possible in eval script to define more than one collection set and do some processing on them.
What I want to try is to create a maximum from selected 3 images per mont - one for each 10 days of the month with the least cloud coverage and then to make a month composite out of this 3.
I suggest you look into the Data Fusion capabilities of Sentinel Hub. There are also several examples of evalscripts on the custom scripts repository such as this one you should find useful.