ok - I found out how to do it by ahem .... reading the docs

The trick is that every empty line after a the last element in the list, demotes next element one level.
(the color-roles are only there to make the nesting clear, they can be removed)
[.red]
Definition-list 1 ::
+
Text in def 1
+
Text in def 1
+
[.blue]
Definition-list 2 :::
+
text in def 2
+
[.green]
Definition-list 3 ::::
+
Text in def 3
+
Text in def 2
+
Text in def 1
Text not in a list