Combining single line text fields and autonumber field

Combining single line text fields and autonumber field

I'm in need of combining 2 single line text fields and 1 autonumber field into a new field.

The single line text fields just need to combine the first letter from each field together with the full auto number field.

Example:
Text field 1: Ext
Text field 2: New
Autonumber: 1234

Combined output into new field: EN1234

I found this example on How Combine 2 fields into One. But is is only part of the solution, I'm stuck on pulling the first letter only from the single line text fields.
https://help.zoho.com/portal/en/community/topic/combine-2-fields-into-1