As far as I know, no one has offered or intends to offer a three-passenger rear seat conversion for the Element.
As far the aftermarket is concerned, the potential liabilities are huge; the market is very small.
Anyone needing to seat three people in the back would have bought a CR-V instead.

And since the Element goes out of production at the end of the year, I would not expect to find a solution.

If you must fit three children in the back seat, the CR-V is the only compact Honda SUV that can do it.
But if more than one of them requires a child seat or a booster, there may not be enough width in a CR-V.
You may need a Pilot instead.