How to say 三つの?
 

Hi. Should we pronounce 三つの as "mitsu no" or "mittsu no"?

It is always pronounced みっつの.

Forget romaji if you are already using kanji, let alone kana. Using romaji will only slow down your Japanese learning process.

The correct kana for "Tokyo"　≠　ときょ
The correct kana for "Tokyo"　＝　とうきょう

This example alone should show you how useless romaji can be.　Unless you type T-O-U-K-Y-O-U, you will NOT get the kanji 「東京」.