Can I use a WQHD 3440 x 1440 monitor on the MacBook M1 Pro?

Is that a specific brand, or a connection spec?

Generally you can attach most monitors to a Mac, assuming you have the right connector.

MacBook Pro (14-inch, 2021) - Technical Specifications - Apple Support

Display Support

Simultaneously supports full native resolution on the built-in display at 1 billion colors and:

• Up to two external displays with up to 6K resolution at 60Hz at over a billion colors (M1 Pro) or
• Up to three external displays with up to 6K resolution and one external display with up to 4K resolution at 60Hz at over a billion colors (M1 Max)

Thunderbolt 4 digital video output

• Native DisplayPort output over USB‑C
• VGA, HDMI, DVI, and Thunderbolt 2 output supported using adapters (sold separately)

HDMI digital video output

• Support for one display with up to 4K resolution at 60Hz
• DVI output using HDMI to DVI Adapter (sold separately)

I currently have two 4K external monitors attached to my 14" MacBook Pro (M1 Pro CPU chip), plus the laptop screen is also open and displaying content.

One is a DELL 4K 32", and one is a Acer 4K 28" monitor.

Yes, you can connect it and it will work.

But whether or not you can get an actual 3440x1440 resolution remains unanswered.

There was a similar question recently by someone who said they couldn't select 3440x1440 in display setup, but it was apparent in the person's screenshots that he/she had set a 30Hz refresh rate but the monitor spec called for a 100Hz refresh rate. I pointed this out but the person who posted has not responded, so we don't know if it was a user problem or an actual display problem.