1.Are => and -> used in the same way?
yes.its just a way of representation.Arrow is in one direction is one way implication.Dont get confuse between single and double line.
2.=> is logical implication and <=> is equivalence right?
Yes.=> is logical implicatin.it implies in one direction ,while <==> implies both ways.
3.Then why does in some questions, => and <=> is read as 'if then'? '->' symbol is for if then right?
a:)If p then q is same as p->q ,it does not mean then q->p.its goes one way.One is symbolic representation,other is English.
b:) If p then q is not p<==>q. .
Refer rosen book.In first chapter they have mentioned around 15 ways of different representation of these symbols.