Sum of last 3 numbers in a range with gaps.

[donjoe]

I have a range where some cells contain numbers and other (random) cells are empty. Can I calculate the sum of the last 3 numbers in that range with a single formula? What would such a formula have to look like? (Preferably, the formula will continue to work if I later expand the range and add new numbers and new gaps at the end.)

Thanks.