can the Apple watch series 3 be used with a CSpire iPhone 7 Plus?

It would not slow the watch down.

When activated on your iPhone's service plan, the built-in cellular capability on Apple Watch Series 3 (GPS + Cellular) enables certain features to remain available when using your Apple Watch away from the paired iPhone, when they would not be available on other models.

The functions that are available on your Apple Watch when it is away from your iPhone depend on various factors including:

• Your Apple Watch model:

  Apple Watch - Compare - Apple

• The features of your carrier service plan:
  • Apple Watch - Carriers - Apple
  • Make a call with Wi-Fi Calling - Apple Support
• The type of connectivity that is available to your Apple Watch at the time:

  About Bluetooth, Wi-Fi, and cellular on your Apple Watch - Apple Support

Features and requirements that apply under various circumstances are detailed here, including within the footnotes:

How to use your Apple Watch without your iPhone nearby - Apple Support

Hi

Apple Watch Series 3 (GPS + Cellular) is not locked to any specific carrier and has a built-in eSIM that copies information from the iPhone's SIM card.

The built-in cellular features require a carrier plan for your iPhone that specifically includes support for Apple Watch Series 3 (GPS + Cellular), including the carrier activating the device on the plan.

Apple does not currently list C Spire as a supported carrier:

Apple Watch - Carriers - Apple

You may wish to contact your carrier to ask about their plans for supporting the feature.

Apple Watch Series 3 (GPS + Cellular) can also be used without activating the built-in cellular feature, in which case it functions like a GPS-only model, in conjunction with an iPhone 6 or newer model of iPhone (running the latest version of iOS) and a service plan from any carrier.

That is correct (except that it is for your carrier to support the built-in cellular feature, not Apple).

The Apple Watch would function like a GPS-only model in the meantime.