It's not unusual for beta software to come with quirks, and we've recently discovered yet another in Android Q Beta 3: RCS messaging functionality previously present in Messages on Pixel 3 phones seems to have disappeared. RCS-enabled conversations have reverted to SMS, and the Chat features option has vanished from the app's settings.
Considering the maddeningly narrow rollout of RCS features on the whole, statistically, this probably isn't an issue you'll encounter before it gets sorted out. We've currently only confirmed the issue to affect Pixel 3 and Pixel 3 XL phones operating on Verizon, although it's possible other phones could be experiencing it, too.
So on the off chance you were previously enjoying RCS messaging and have lost the functionality since updating to Q Beta 3, just wait it out. Like most beta bugs, this one should be ironed out in a future update.