Wa'alaykumsalam Wa Rahmatullah.
The last third part of the night is calculated as such.
The number of hours between Maghrib and Fajr. If it is 9 hours, divide them by three... this means that 3 hours prior to Fajr is the last third part -
4;10 am is Fajr, therefore the last third part of the night starts 1;10 am
Allah knows best.