If you're not going to change sprockets, you have to look at the gear ratios of the hubs. For instance this 3 speed hub

https://www.sturmey-archer.com/en/products/detail/s-rc3

has a lower gear ratio than this 8 speed:

https://www.sturmey-archer.com/en/products/detail/x-rd8

A more useful way to thing about is: the hub gives you a range of ratios, and the sprockets determine where that range is located. You can get lower gears with your existing hub just by changing the chainring (front sprocket).