A textbox, by definition, contains text. Also keep in mind that
"0" is not equal to 0
The first is a string containing one character, the other is an integer. Everything is stored internally as numbers. The number zero has a value of (you guessed it), 0. However the character "0" has a value (internally) of 30 (unless you are using unicode but let's not worry about that now). How it is interpreted depends on the type of variable used to reference it.
Short version - if you want to check a textbox and replace zero with one you have to do
if (textbox3.text = "0") then (textbox.text = "1")