From VG247: “One of the Week 3 challenges involves eliminating your opponents while using the Slipstreams, so lets take a look at where to find them.
Slipstreams are one of the new additions to the Fortnite map in Season 9, and offer a new mode of transport to get around the map quickly. As part of the Week 3 challenges, you must eliminate three opponents within 10 seconds of using a Slipstream.”
Source: N4G PC Fortnite: all the Slipstream locations