Check the translation into English or Russian.
1.C'est une affaire moins grave.
This case is not so important.
Это дело не такое серьёзное.

2. Cette forme d'amour est pire que l'haine.
Such demonstration of love is worse than hatred.
Такое проявление любви хуже ненависти.

3. Jean est un de mes meilleurs amis.
Jean is one of my best friends.
Жан - один из моих лучших друзей.

Question

Check the translation into English or Russian.
1.C'est une affaire moins grave.
This case is not so important. 
Это дело не такое серьёзное.

2. Cette forme d'amour est pire que l'haine.
Such demonstration of love is worse than hatred.
Такое проявление любви хуже ненависти.

3. Jean est un de mes meilleurs amis.
Jean is one of my best friends. 
Жан - один из моих лучших друзей.

Answer

Yes, I'd agree on all those translations. For the first I'd say 'This is a less serious case/affair/business' (depending on the context' ), but what you've said is fine. For the second, I'd just say 'This form of love ..' . rather than 'demonstration.' The last one is fine.