In the 1st case electric field lines are curved and in the 3rd case electric field lines are straight. What are the conditions for such to take place?

In the first and the second case we are talking about two individiual charges and thus the field lines between them are exactly like between the two poles of the magnet(which is why they are called electric dipoles). in third case these are a stream of positive and negative charges. each charge behaves the same but when u see all the charges together in a straight line the curved field lines tend to cancel out(only theoratically) and thus only the straight field line is shown. Try imagining multiple dipoles in the first case.....