Definitely another 16gb 3000mHz stick
The "ryzen needs faster RAM" thing is overblown. If you had "slow" RAM by today's standards (<=2400) then yes, you may see a benefit with Ryzen, but a small benefit (some increased performance depending on program, increased FPS in 1080p gaming, almost no difference at 1440p or 4k gaming). Certainly not worth ditching a 16gb stick of 3000Mhz ram, when you could just add another stick, have dual channel, and have 32 gb of 3000 mHz ram. You will be golden with that.
The returns on upgrading RAM speed for ryzen diminish when you: get to 3000 Mhz, and go beyond 1080p. You are already at a good speed, get another identical stick, your 32 gb 3000mhz, 3900x system will be a champ.