It's actually just Modo arigato.

No idea what three, nine, modotachi means but it is not "thankyou" in Japanese. There are five ways to say "Thankyou" in Japanese from casual to formal:

• "Domo" or just an informal "thanks" said mostly in close friend circles
• "arigato" or just an informal "thank you", but better than "Domo" thanks outside friend circles
• "Domo arigato" is the in between informal and formal, reinforcing the power of "thanks"
• "arigato gozaimasu" a more formal "thank you very much" to people you do not know and possible higher professional status.
• "Domo arigato gozaimasu" reserved for known people of higher status and a very very formal "thank you very much".

Source: spent 8 years in Japan and can speak a bit of the language.