I have Pro timer 3. I'm doing astro and wonder if I using this right. I'm shooting a 20 second exposer and want a 10 Secord interval. I'm setting the timer for 30 seconds. I started the timer the shutter opens then closes in 20 seconds. At he end of the final 10 seconds of 30, the timer repeats the first run until I stop the pro 3. Is this a correct way to run the timer?

The "interval" is always the time from one shutter opening to the next. This means, it includes the exposure.
So what you want is a 30 seconds interval and a 20 Seconds exposure.

For any exposure times shorter than 30 seconds, I'd recommend not to use Bulb but to configure the Exposure time in the Camera (M Mode) and just use the Timelapse (M) mode. For longer exposure times you can use TL Astro mode and set the camera to Bulb - then you'd need to configure Interval and Exposure in the LRT PT.

But always remember: Interval = Exposure Time + Dark Time!

Also make sure to check the manual for further explanations: https://lrtimelapse.com/lrtpt/manual/

I'm not using Bulb mode. I using the camera to set the shutter time and the pro timer to cycle the shutter and the dark time. For each shot.

That's perfect.