No car pack DLC will be released for Forza Horizon 3 this month, according to Turn 10 Studios' community manager.
As stated by Brian Ekberg on the official Forza Motorsport website, the focus on the recently-announced Hot Wheels Expansion for May 2017 means there won't be a separate car pack released for Forza Horizon 3 as well.
Therefore, May 2017 will mark the first time since the game's launch in September 2016 a monthly car pack hasn't been introduced to Forza Horizon 3.
However, there will be additional content coming to Forza Horizon 3 in May outside of the Hot Wheels Expansion. On top of the Porsche 356 Speedster that'll be introduced as a limited-time barn find, Forza Horizon 3's weekly Forzathon challenges will allow players to win the Porsche 911 GT2 RS, the Porsche 911 (991.1) Turbo S and the Porsche Carrera GT by completing specific tasks.