The license goes from the date of purchase, so the new software will expire simultaneously on all 3.
In other words, it doesn't matter when you install it.
Sorry, post amended. not sure if you were already looking at it.
I'm sorry I think I made a mistake in my first answer - duly amended. If in real doubt the final answer would come from Customer Service, a free phone call or online chat, and linked under Useful Links at the top of this page.
You have 3 licenses to install, so you can install the McAfee in one of the machine which needs immediate coverage
If requires you can install the McAfee in all the 3 machines at the same time , or you can wait and install the McAfee in the other 2 PC’s
All the 3 licenses will expire at the same time because it is a package
To get a clear information you may also call customer service as per peter’s suggestion