How do I insert a specified statement at a specific place on each line?

I have a bunch of numbers
3235235
4657456
3454656
4636546
3435457
3463546
I want to insert after the third character of each line, order_no:
becomes
323 no.

Mar.07,2021

two powerful text processing tools under unix awk sed understand (not much research on,:) escape)
most programming languages can also be judged by regularity and logic


I can use notepadPP.

MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1ea06a0-1ad9.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1ea06a0-1ad9.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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