1. You need to resample.
2. You can automate the process.
1.
Open one of the images.
Go to Image > Image Size
Set the units to Percent (100)
Change the resolution to 300 ppi (not cm!)
Make sure Resample is checked
2. automation
Test the above steps to make sure it works.
Record the steps as an action.
Make a droplet from the action.
Drag the folder with the images to the droplet.