I have discussed my views at length in
this thread: PTSB Mortgage holders to get 10pc bonus for paying lump sum off trackers: I suggest that you read it in full as it gives both sides of the argument about negative equity mortgages.
Cannot afford to pay neg equity
...
(2 strong incomes).
If you have such strong incomes, you should be looking to eliminate your negative equity as quickly as possible before considering buying another house.
Taking on another mortgage would mean that you are overborrowed.
If you need to move in three years, then rent and keep your home as an investment property. With a cheap tracker, it might well be a good long term investment.
If I was in your shoes, I would take advantage of this offer and pay down the mortgage as much as possible. Keep saving and pay off as much as possible before the deadline. It might be worth borrowing from friends short term to pay off more.
If you are determined to expose yourself to further borrowing in three years time, then you should not take advantage of this deal. You will need as much savings as possible to have a good deposit.
By the way, I would not be as confident as your broker, that you will be able to get a mortgage in three years' time, while you still have substantial negative equity on another property.