Interesting question. If you have a 110 or 130 bcd crank set it is fairly easy to add a second chainring , say 38 teeth, that you can. manually shift if needed. Chain rings at vuelta are not expensive. If I were deciding between the 3 and 6 speeds I would check the overall condition closely, particularly slop in the rea,r triangle hinge and smoothness in the headset. The Brompton 3 speeds both get above 80 gear inches which is more the issue with small wheels than getting low gears, at least for me.