What happening is most likely you already closed your form and can't
open it again.
Assuming, you have both instances well and alive
In Form2 you should have
And In Form3 you should have
It can be something like this
shared sub Main
dim f2 as new Form2()
dim f3 as new Form3()
f2.Next = f3
f3.previous = f2
To link forms you creating properties,
And then use that as way to operate the form that should open
In form code do
private sub BtnNext_Click(....).....
and the same way for the previous. If you have wizard, you could chain
all your forms this way.
and of course, to accomplish this, minimum, you need an interface that
contracts your forms to implement Properties
Previous or you can have a base class with implementation of
the buttons and properties and then it will all work.