Kif,
The black Singer 221 and 221K Featherweights were made from 1933 to 1957ish. The white 221K was made from about 1964 to 1968. They don't all have the 221 designation on the front. BUT they all have a flip-up bed extension on the left and the light on the front of the horizontal arm. Those two feature are a great way to spot the Featherweight. The Featherweight is considered to be a half sized machine.
I assume you are asking about the 301, as the Singer 3 is a huge industrial machine from around 1900. The 301 is a full size machine and also has a flip-up bed extension. The motor on the 301 is internal and gear driven. On the 221 the motor is on the rear and drives the machine by a belt.
The 301 and the 221 both use the same bobbins and bobbin case.
Cathy