Is there a dedicated camera is for recording video on iPhone 12 Pro? I am going to get an iPhone 12 Pro to use as a high quality video camera. I know the 3 cameras have different functions, but I don’t know much about this triple camera setup.

Is there a dedicated camera to record video, or does it use all 3 at a time for maximum quality?
If it uses just one camera at a time, which one is the highest quality?
The built in Camera app on your iPhone can shoot video from one camera at a time. If you'd like to shoot with multiple cameras at a time, third-party apps can enable this functionality since iOS 13 - for example, DoubleTake by FiLMiC Pro. This does not improve the video quality, but it can let you capture with both the front-facing and rear-facing camera at the same time, for example.
To answer your second question, these are some of the features available on iPhone 12 Pro based on which camera you use:
Available for photos only

Night mode (Ultra Wide, Wide)
Deep Fusion (Ultra Wide, Wide, Telephoto)
Smart HDR 3 (Ultra Wide, Wide, Telephoto)
Apple ProRAW (Ultra Wide, Wide, Telephoto)
Lens correction (Ultra Wide)
Portrait lighting (Wide, Telephoto)
100% Focus Pixels (Wide)

Available for photos and videos

Optical image stabilization (Wide and Telephoto)
f/1.6 aperture (Wide), f/2.0 aperture (Telephoto), f/2.4 aperture (Ultra Wide)
Five-element lens (Ultra Wide), six-element lens (Telephoto), seven-element lens (Wide)

Available for videos only

Sensor-shift optical image stabilization (Wide)
HDR video recording with Dolby Vision (Ultra Wide, Wide, Telephoto)
Slo‑mo video support for 1080p at 120 fps or 240 fps (Ultra Wide, Wide, Telephoto)

Though all of the cameras perform well enough in great light, it's my personal experience that the Ultra Wide camera especially suffers in dim environments. You should pick which camera you'd like to use based on what you want to capture - the camera's field of view and lens distortion can drastically alter the look of an image. Compare these 3 images, all shot on iPhone 12 Pro, keeping in mind that Imgur compression has crushed the fine details:
Telephoto

Wide

Ultrawide

Sources

https://www.apple.com/iphone-12-pro/specs/
https://www.photographyblog.com/reviews/apple_iphone_12_pro_review