Do you really want to trade your collateral position for something less? Remember to forecluse on the 3rd you will assume balances the 1st and the 2nd. When you consider those balances and all the time and expense that would be required to collect your money via foreclosure, are you prepared to do so.
IMO it is not ecomomicic or purdent to take a lesser position. I would also take a hard look at borrower financial condition. 3 mortgages leaves what kind of a equity in the property? They have pledged another liquid assest in form of stock. Do they want the stock released so they pledge it on another loan somewhere else? Collateral isn't the only thing to consider as the borrower must show the ability to pay off the loan without looking to liquidate collateral.