I want to upgrade from bit Mojave 10.14.6 to Montery but have the following questions and concerns:

I would personally not worry about upgrading that old Mac to Monterey. You won't gain much but you could encounter a host of incompatibility problems due to the aging hardware.

As @Nancy OShea says, get a new Mac when you really need the latest macOS (currently Ventura) or your current Mac eventually stops working.

you won't be able to use ca3 on any other computer. ie, you can't reinstall and use it beyond the 30 day trial period.

your data (eg, photos can be transferred without problem).

Get a new computer. If you're still using CS3, your computer hardware probably isn't sufficient to run Monterey or Ventura.

OPTION 1: Backup your work files to an external drive or USB stick. 

OPTION 2: Use a dedicated backup & recovery service like Blackblaze that automatically copies files to the cloud each night.  When you migrate to a new computer, you can easily recover your old work files to the new computer.

NOTE:  CS3 is dead.  It cannot be installed or activated on a new computer.

MODERN OPTIONS:
Photoshop Elements (no subscription needed).
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• 20 GB cloud storage (upgradable to 1 TB).
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Creative Cloud All Apps (51 + pro apps & services) and Single App Plans
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