I have an old Suteki beater bike with a shimano 170 crank set. I was interested in possibly lowering the gearing a bit, currently has 52 and 42 rings. also interesting in having a 3 ring crank and 175 pedal arms. Are all shimano's cranks compatible or are there groups of compatibility? Did not know if it is doable and what to look for if it is. bike also has Shimano 600 derailleurs, if that is needed for info.
You can change to a triple. You will need to change the BB to accept the added length of the extra chain ring. Also will need to change the front and rear dérailleurs.
Shimano has made many different crank designs over the years but they are typically interchangable within groups of the same period.
Your current crank is probably a square taper bottom bracket design and you may be able to fit smaller rings. How small will depend on the "bolt circle diameter" of the chairring bolts (130 mm or 110 mm). A 130 mm bcd will accept an inner ring down to 38T and a 110 mm bcd will take down to 34T.
A new triple crank could be fitted and will most likely require a new bottom bracket so the two will have to be compatible for design and length. For really low gearing you could even use an MTB crank which are usually geared 44/34/22.
The current derailleurs may be usable but it will depend on the crank and gearing you choose. Your rear derailleur has a better chance of being usable.