Hi,
Thanks for using the HP Forums.
Yes, the Officejet K7100 driver is available in Win 10 IN_ OS drivers. Follow these steps to install:
Win 10 In-OS drivers
- Right-click on the Start button (Windows icon) select "Control Panel"
- Under "Control Panel", select "View devices and printers"
- Select "Add a printer"
- If it finds your printer (assume you have your printer plug in via USB on your computer), follow the steps to install the In-OS drivers. Else, select "The printer that I want isn't listed" > Next
- In the next screen, select "Add a local printer or network printer with manual settings" > Next
- Select "Use an existing port:" > Next
- Click on "Windows Update" Note that the "Windows Update" may take up to 10 minutes depending on how fast your broadband speed is"

- Click "Next" and follow the remaining steps to install your printer driver.
Once the in-OS driver is installed, check to see if teh A3 option is available. If it is not, try creating and use a "Custom paper Size" for your printer. This resource will show you how to do that :
Creating a Custom paper Size for your printer
Hope that helps!
I am an HP Employee. Although I am speaking for myself and not for HP.
Click Helpful = Yes to say Thank You.
Question / Concern Answered, Click "Accept as Solution"