I don’t see anything unusual about your memory usage.
It’s even a pretty small instance actually. Larger setups consume more (like mine to use ~750MB with ~550MB resident, also on Pi3).
Don’t stare at the ‘free’ value, it is actually meaningless as the OS uses all but a defined minimum of the available capacity for caching but will release that immediately whenever any app requests memory.
As you say you encounter freezes, find out first if that’s due to memory. Run iostat 2 or the like to see if the system starts paging when it gets slow. It could well be due to other reasons such as SD corruption.
You didn tell us your OS specs, please follow How to ask a good question / Help Us Help You - Tutorials & Examples - openHAB Community next time you post.
Do you use openHABian ? If no move over there and enable ZRAM, that’s a memory optimized setup.