combination of three fields make as coalesce in transfer map script?

[Arsshad1]

Hi Experts 
Good day!
I have one requirement as below:
In data source transfer map I have 10 fields in those fields the combination of three fields I want to make as unique means coalesce.[ Three field combination same records not to be created/Inserted ] But as my knowledge coalesce "true" I can make for individual  fields. But for this scenario how can I achieve this ?
if it is script like "on after" "on before" Can someone please help me with the script.
Fields are : 1.Customer 2.UCIDN 3.Material ID
staging table : u_material_im

Targer table: u_mat

Any help will be appreciated 

Thanks in advance 
Arsshad

[Kieran Anson]

ServiceNow documentation contains an example for this, called 'Conditional Coalesce'

Updating records using coalesce (servicenow.com)