@bdipti,
It's a big nightmare for a child to see his parents divorcing.
Any child would be very anxious if his parents were divorcing & his/her father was having another relationship.
Think it this way: If you were of his children's age, & you came to know your parents were divorcing & your father was having another relationship, how would you react?
Not only his three children, but your partner himself loves his children like anything.
His family actually needs him. It's not easy for a mother to look after & care for her three children & at the same time emotionally support them.
Your partner has left a big responsibility of himself.
Honestly, if I were you, I would tell him to finish that relationship & go support his family.
If you really love him, you'll understand my point of view.
He's in a big trouble himself. On his one side are you, & on the other his family.
I'm 14 myself. I get anxious for my parents arguing on little matters. And I think I'll get an attack if my parents end up in such a position as that.
His family needs him more than you, & he loves his family as much as, or I guess more than you.
Choice is yours.