I owned an ADM3a and used it often back in the days of 300 and 1200 baud modems. My recollection is that it was just a dumb TTY.

The ADM3A was slightly more than a dumb TTY; it supported direct cursor addressing despite being made of SSI TTL chips. It was the main platform for vi. I bought a used one at a government surplus auction and, many years later, wrote an emulator. Thanks to termcap/terminfo, most TUI programs work adequately on an ADM3A emulator.

No graphics, though, not even TRS-80-style pseudographics. Not even inverse video.

The ADM3a was just a UART chip, RAM chips, character ROMs, and a bunch of 74LS chips to glue it all together. No CPU, and no way to generate graphics.

Just to clarify, I think a lot of the Tek terminals that could generate graphics also lacked CPUs.