In the name of Allah, Most Gracious, Most Merciful
Assalaamu `alaykum waRahmatullahi Wabarakatuh
In principle, divorce to a pregnant woman is valid. If the husband issued the third divorce to his pregnant wife, it will be valid and that will constitute a talaq-e-mughallazah (a permanently irrevocable divorce)
And Allah knows best
Wassalam
Mufti Ebrahim Desai
Darul Iftaa, Madrassah In’aamiyyah