There are changes the dealer can make in addition to picking a set of parts chosen at the factory level .
2 is a freewheel
Shift 2 | Shift | Recreation bikes | City bikes | Bikes | Trek Bikes
3 uses a cassette-freehub (1 more 'speed') a bump up in the components like the crankset
Granny gear may be bolted rather than riveted on, so excangeable for smaller*.
Shift 3 Women's | Shift | Recreation bikes | City bikes | Bikes | Trek Bikes
* Less teeth , the rear gear cassette can also be exchanged at point of sale by the dealer, for larger to crete a lower gear ratio..