USB to 3v converter to power a battery powered FM transmitter?
It won't go "poof" immediately, and will probably not go "poof" at all.

The transmitter has to be able to handle 3.3Volts because new alkaline batteries will have a voltage higher than 1.5Volts.  A new battery may well reach 1.65 Volts, so two in series is 3.3Volts.

Since AAA batteries won't manage to put out 1.65Volts for very long under load, there is a slight chance the transmitter isn't designed to handle that voltage for very long and that it might over heat and burn out.  I don't think it likely, but there's no way to tell without looking at the device or the schematic.

You might get more noise when powering the transmitter from USB with a switching regulator (which a buck regulator is.)  There will be noise on the 5Volt and ground lines from the USB port, and there will be more noise from the regulator.
How much more noise you will hear depends on how well made the transmitter is.  It might have good filtering/regulation for the power supply and you won't hear any noise at all.  It might straight up suck canal water and manage to not only transmit all the USB noise but amplify it as well.  Given the price and that it is designed for batteries, I wouldn't make any bets as to what will happen.