However, cost for the reservation instance is static for entire term.
If the VM's are being active approximately for 10-12 hours in a day, then I would suggest you to use the Auto Start and Shut Down technique to save on cost.
I suggest you to go through the Azure Pricing calculator to find the cost for your VMs based on your usages and check the Reservation savings too.
Below is an example for a VM for your reference:
If a VM is used for 12 Hrs for 30 days - 360 Hrs per month (you can exclude weekends depending on the actual usage which will get down the hours still down) you will pay per VM as shown in the screenshot:
If a VM is reserved for 1 year, the charge per month is as estimated below:
If a VM is reserved for 3 years, the charge per month is as estimated below:
Hope this information is helpful for deciding the way for better cost saving on VM's.
Please 'Accept as answer' if the provided information is helpful, so that it can help others in the community looking for help on similar topics.