use "${3|one,two,three|}" for choice and not eg. ${1:NO_ALARM }${2:MINOR }${3:MAJOR}
https://code.visualstudio.com/docs/editor/userdefinedsnippets
Choice
Placeholders can have choices as values. The syntax is a comma-separated enumeration of values, enclosed with the pipe-character, for example ${1|one,two,three|}. When the snippet is inserted and the placeholder selected, choices will prompt the user to pick one of the values.
https://code.visualstudio.com/docs/editor/userdefinedsnippets Choice Placeholders can have choices as values. The syntax is a comma-separated enumeration of values, enclosed with the pipe-character, for example ${1|one,two,three|}. When the snippet is inserted and the placeholder selected, choices will prompt the user to pick one of the values.