I'm not sure what determines the value of ASIL. But for your example you can write a if then else statement.
To write a statement with an enumeration you would use ModuleName.EnumerationName.EnumerationValue. Most of the time if you press ctrl space and type the name of the enumeration you can find it in the suggestion box.
Here is an example with an enumeration called "Enumeration"
Heres an example for your use case
Whatever entity that has the attributes of your enumerations would replace "$object"
if $object/controlability = Modulename.Controllability.c3 and $object/saverity = Modulename.Saverity.s1 and $object/exposure = Modulename.exposure.e3
else //more logic
Hope this helps!
Edit: Your microflow will have one parameter. The parameter would be an object of type Hara. Then you would use a change activity for Hara and change the attribute ASIL. For your expression when changing the attribute follow this strcuture
if //conditions for A
else if //conditions for B
else if //conditions for C