I would:
1. Set up your test grid, with your 3 images ( or however many ). Get the styling right eg, grid container padding, grid gap etc.
2. Set your Responsive grid widths for Tablet and Mobile. Again adjust any container spacing, grid gap.
3. Once you’re happy with the layout – use the Web Inspector to inspect the images. You will want to inspect the size of the image in the developer tools for the various device widths: Desktop / Tablet and Mobile by reducing the browser width.
Whichever device has the largest image, generally that is Tablet view if the grid columns are set to 100%, is the max image size you require.
Final thing – if you’re not too bothered about bandwidth then you can double that size to account for x2/Retina screens.