Why ban all three? Multiple accounts is not exactly against the rules, although it's frowned upon.
Only the account caught should be banned, IMO
25. Ban evasion (using or creating accounts while one of your accounts is banned) is not allowed.[e]
Although this rule is for after the ban when they create another account. In these case the use already have his alts. Perhaps a call from the community will be interesting.
update:
...using...
Ok, seems like we have our answer. All three can be banned if one is banned.