I have a similar Sugino crankset to which I added a third smaller chainring. The chainrings I used are 52T, 40T and 28T. You will likely have to use a longer spindle to get the proper chainline and use a front derailleur designed for a triple; you may or may not need to change the rear derailleur depending on its cage length. Also, make sure that the small chainring you use has no more than 12T difference with the middle chainring for good shifting. I found that I could use a 28T with a 40T, but when I tried a 42T in the middle, the 28T was too small (so I'm now looking for a 30T to use that 42T).