Elias here with Shopify Support. Thank you for reaching out.
I understand you're using the Venture theme and you've set the number of products to 3 per row; however, it's only showing 2 products per row on the mobile view. Just to confirm, does the desktop correctly display 3 products per row?
I can confirm that this is not a coding error, as the 3 products per row setting is only for the desktop view. Though it's possible to customize your theme to show 3 products per row on mobile, this is not something that we'd recommend, as the product images will appear much smaller and the page will feel more cluttered. Products with longer titles will also appear crammed when displaying 3 products per row (on mobile).
That being said, if you'd like to move forward with adjusting the number of products shown per row on the mobile view of your theme, this is something that our Theme Support team can help you with. Since Venture is a Shopify-supported theme and, if your store is subscribed to a plan, you're eligible to use the complimentary design time included with your plan for this customization. While we're unable to create customization requests via the Community Forums at this time, we'd be happy to continue assisting you through live chat, email, or callback. Please visit Shopify Help Center and log into your account to create a support request.
I hope this helps! If you have any other questions on this topic, don't hesitate to let me know!