Ideally, you can fill the S type airlock with water and 3 piece airlock does not need any water to arm the lock. However many brewers recommend filling the airlock with disinfectant (sanitizer, or spirit). This is because:
If there is a drastic temperature change, there are chances that S type airlock will start sucking back air into the fermenter. A sanitizer will ensure that microbes are not introduced in the process. If the headspace is not enough, the fermenter can overflow. This causes yeast and organic material to spill out and attract flies/contaminants. A sanitizer limits the damage. One big problem with fermentation is flies get attracted to the CO2 being generated. They often lay eggs in the airlock.