switching cranks
 

is it possible to put a 3 peice crank on a bike that originaly came with a one peice

Yes. Just make sure you get the right type of bottom bracket. Having a one piece crank, I'll assume it's an American bottom bracket.

Providing the type of bike, original cranks and upgraded cranks would help. Most likely the one piece cranks were an American bottom bracket (with cups) and your new cranks will need a 19mm or 22mm mid bottom bracket (without cups)

Very much false. An American bb, by definition, is a Mid sized bearing inside of a cup to fit the frame. So if he switches, he still has to have an American bb, meaning Mid's in cups pressed into the frame. You just sorta canceled out your own statements/logic in the parts I highlighted...

Because you are correct (an Amer BB is a Mid Bearing w/in a cup), he can simply order/purchase his crankset and a Mid BB and simply re-use the cups. Very rarely have I seen the need to replace cups when maintaining/upgrading Amer BB's. However, I frequently see the need to replace/upgrade the bearing set during the same process.

The bearings in a 1-pc crank are unsealed, meaning that the inside of the cups are rounded for the open balls. With a sealed bearing, its squared off, therefore the cup won't allow the new bearing to fit, so he needs to order an all new American BB.

If I was putting a 3 piece crank on a bike that had a one piece crank I would use a sealed bottom bracket. However, many bikes come with 3 piece cranks with an unsealed loose ball american bottom bracket. All the Nitrous, Kink Curb, Stolen Casino, just to name a few. They do use a different spindle which won't work with a sealed BB, but since you get a spindle when you buy a set of cranks, you just need a sealed american bottom bracket and the set of cranks.

You are in good company there are many different conversion kits,
to put a smaller bearing assembly , in a larger as it is frame portion, the BB shell.

Check with your local Bike shop, and see what they can order , from their wholesalers,
as well as, what they have in stock.

I never thout about that. good advice