I've been thinking about going to a triple lately, so I've been looking into this a bit. It appears there are three possibilities.

1) Buy a TA tripilizer to replace you 42T ring. The tripilizer is a 144mm BCD 42T chainring that has extensions that allow you to bolt a 74mm BCD chainring to it. You can get a 74mm TA chainring a small as 24T.

2) Buy a newer triple crankset. I've read that some people bought a Campagnolo Mirage crankset for that purpose. Those are realatively cheap. Others may work, but are expensive.

3) Buy a NOS or used Campagnolo triple crankset, if you can find one.

You'll probably need a longer spindle to make room for the third chainring and maintain chin alignment. I think the longer spindle would fit your bottom bracket, but I'm no expert.