Hello!
Where and to a certain extent when (especially to anyone else who might be reading this months or years down the road) you decide to migrate will affect what options are available. While a Pay-as-you-go offer can usually do whatever you need there is often a better option as there are a lot of offers to choose from for your subscription. Here are a couple tools that are really helpful in determining what you'll need:
Azure Pricing Calculator
https://azure.microsoft.com/en-us/pricing/calculator/
This allows you to configure all azure services for your specific scenario and get an estimate of what it will cost, including Pay-as-you-go pricing versus reserved pricing.
Azure Sales
https://azure.microsoft.com/en-us/overview/contact-azure-sales/
There are a lot of different options when it comes to which offer you decide to go with, even for your scenario of 3 legacy Windows 2008 VMs. Sales will be able to walk you through the benefits of each offer type to match what works best for your scenario (a Pay-as-you-go subscription would work but there are several others which may offer better benefits for you).