In fact the word add "X instead" is not equal to prevent. From the rulebook: "When a card uses the phrase "instead" or "instead of", the stated effect is replaced with a different effect. This does not mean that the original effect is prevented. If the second effect cannot happen for any reason, then the original effect occurs.".
So i guess it won´t fall in the "preventing" issue, and yes, you can add 2 burdens instead of 3.