Q:

Is the correct way of praying the third rakaat of isha since I saw it online?

Bismillah, Alhamdu , Bismillah , surah , alahu akbar, Dua qunoot

Then when going into ruku not saying alahu akbar?

A:

In the Name of Allah, the All-Merciful, the Mercy-Giving.

All praise is due to Allah. Peace and blessings be upon His Messenger Muhammad. 

I wonder where you got this information from.

The third rak`ah (Prayer unit) of `Isha Prayer bears no difference from other rak`ahs. They are all the same. There is no qunout (supplication) before ruku` (bowing) in the third rak`ah and ruku` should be preceded by saying Allahu akbar.

Allah knows best.