Hum Im not sure Im getting you here… Have you looked at Image Processor…?
Why are you NOT just using Fit Image and canvas size in your actions…?
These are all built-in to Photoshop.
If you wanted to do all 3 sizes in the 1 fly-bye then use script to process…
If you need extra file naming conventions then script would probably be best…
All of the above should have NO trouble handling your 3k files…