I have defined 4 trigger prototypes on same item (vfs.fs.size[{#FSNAME},pfree]) with different thresholds and severities set as Warning, Average, High and Disaster (see trigger-prototypes.png).
It should be possible to define a dependency between them but there is no Dependencies tab in CONFIGURATION OF TRIGGER PROTOTYPES screen.

specification at https://www.zabbix.org/wiki/Docs/specs/ZBXNEXT-1229