Need to consider category first 3 letters in item rule set for item code generation

Summary:

 Greetings!

I am trying to add substring in my item rule sets it is giving me an error.

My condition expression was - substring(Catalog[Main_Catalog].Category[MOB.01.FLS],0,3) in secondary condition.

Actually my category code is MOB.01.FLS from here i need to pull starting 3 letters to get into item code rules like MOB only in this case.

Requirement is to have item code based on rule set as below:

 [Item].[Item Code Creation Attributes].[Channel] == "AB'" &&

 [Item].[Item Code Creation Attributes].[Item Brand] == "ABCDEF" &&

substring(Catalog[Main_Catalog].Category[MOB.01.FLS],0,3) &&

 [Item].[Item Code Creation Attributes].[Model Code] == "A23+"