Declare a static string in Form1;
In Form1
public static string SetValueForTextBox1 = "";
and in button click;
SetValueForTextBox1 = textBox1.Text;
similaarly declare another static variable in Form2
public static string SetValueForTextBox2 = "";
and in button click;
SetValueForTextBox2 = textBox1.Text;
and in the form3 on Printbuttonclick, You can get the textbox values from FOrm1 and form2 like,
Form1.SetValueForTextBox1 ;
Form2.SetValueForTextBox2 ;