You don't say whether these are fields, but assuming that by 'text boxes' you mean 'fields shown as edit boxes', you just need to make 'box 3' a calculation field, defined as:
If (box1 = "Hello" and box2 = "Goodbye"; "Hello";"")
box1 = "Hello";
box2 = "Goodbye"; "Hello";
(I prefer to use Case. as it gives more scope for other options.)
Either way, you don't need an empty default result:
// field3 =
If ( field1 = "Hello" and field2 = "Goodbye"; "Hello" )
I also prefer to always use Case(), but've come to see that if() reads more natural.
Sorry for confusing you:
// is a one-line comment sign; simply ignore it, and only use the form shown in the second line.
Does it matter if the text in Box 1 and 2 is from a drop down as it still doesn't work?
No, it doesn't matter (although of course the values in the value list must be correct). I've prepared a demo of what we're trying to explain - maybe we've misunderstood what you're trying to do. Here's a link to the file: Dropbox - calc_test.zip
Apologies for this I had a play around and realised that i'd made a couple of basic errors:
1: the case / if bits you gave me I was placing in the conditional format area.
2: I hadn't set the text box to calculation, instead it was still a text box.
So the boxes work based on the parameters I gave you before, i'm wondering now though, would it be possible for box 3 to reflect what's in box 2 regardless of what that is, as long as box 1 shows "hello".
i.e. box 1 = "Hello"
Box 2 = "orange"
Box 3 = "Orange"
If ( box1 = "hello" ; box2 )
or, for your example
If ( box1 = "hello" ; Proper ( box2 ) )