Don't Buy the Season 3 Battle Pass Because It's Over Soon
Fortnite Battle Royale Season 3 ends on April 30th. That's a concrete end date for the current Fortnite Battle Royale season and that's a major reason not to buy the Season 3 Battle Pass.
Once Season 3 is over you cannot unlock andy of the rewards even if you keep playing. This means you have about two weeks to get through 75-150 hours of challenges if you want everything the Battle Pass has to offer.
The one counterpoint to this reasons is if you've already invested hours and hours into Season 3 and you decided that you have enough tiers unlocked now to get a good amount of gear from buying this late in the season. All the gear you unlock is yours to keep and carries over to Fortnite Season 4.