I cannot partition my external ssd into three partitions

I want to partition my external ssd into three partitions, keeping all my data (300gb) available for macOS, windowsOS and Linux.

First question is what are the correct formats for these three OSs?

Second question, which you can see in the photo is that even after fully erasing the ssd it apparently has 696mb of data already used, what is this?

Third question, the photo shows this, disk utility is not giving me the option to reduce the size of the first partition, why not/how to resolve? (it is greyed out (2.1tb)).

Thank you