Several ways to do it.
But if you want to do it easily
Create public methods on the forms that each can call to pass the value.
Also, in your main form(form 1) create instances(using variables) to display form 2/3.
Then you use the variables.method(passstringhere) or whatever you want to pass.
If you want form 2/3 to pass stuff to Form1, then have a property on form2/3 where you set the parent(which would be a ref to form 1 (me))
And have it call a method on it's parent to set values in form1.
Remember forms are just classes, treat them (almost) no different.
You can also create events etc on the forms, use your variable instances to raise the events passing it values etc.