Hello!
You can find their declension patterns on our website:
For masculine nouns: Declension I
"Игорь" has the same endings as "житель"
"Николай" has the same endings as "герой"
For Наталия: Declension II
This noun has the same endings as "Мария". Note that the name Наталья is declined like "героиня"
Thus, those names will have the following forms
Nom: Игорь, Николай, Наталия
Gen: Игоря, Николая, Наталии
Dat: Игорю, Николаю, Наталии
Acc: Игоря, Николая, Наталию
Ins: Игорем, Николаем, Наталией
Prep: о(б) Игоре, Николае, Наталии