Assuming you tandem has TA Cyclotouriste cranks, you'll need a new six chainring bolt kit for the triple (TA ref 86). They've got longer sleeve bolts and extra spacers for the third ring.

Of course, our good friends at pb*bikes eBay have it at about 5 times its normal value. I've seen them pop up on the 'bay for around $25, like this set that ended a few months ago.

Have you checked to see if your BB spindle is long enough for a triple? The triple will require about 6mm more room than a double, possibly requiring a longer BB spindle.