What’s the difference between the three piece airlock and the S-Type airlock?

There are no significant differences between the two types of airlocks in terms of functionality. Both will serve the purpose just fine. The three piece is much easier to clean, especially with the airlock brush, which was designed specifically to clean the three piece airlock. However, many homebrewers use the S-Type on their secondary fermentor, as it provides an easy method of monitoring the pressure inside the carboy.