J
I can't comment, so using an answer.
What exactly do you mean by "However, when they were finished there were no parallel jobs available in our account." If you use the MS hosted agents, and if you bought three parallel jobs, you should see them in the Organization settings -> Parallel jobs overview. If not, try lo log out, clear cache etc. The purchases are indeed for a month, as explained https://azure.microsoft.com/en-gb/pricing/details/devops/azure-devops-services/ , the costs would be about 40 euros for a month, so for three it would be around 120 euro a month.
As an alternative, and as suggested by others, you could use self hosted agents. They are cheaper, and depending on resources on the host where you install the agent, they run faster.
And then, depending on your needs, there are several ways that might speed up your deployment process, but that usually depends on what you've already configured and your needs.
And then last, if you are a really early user of DevOps, there was an option to pay for hosted agents by the minute. That could explain your situation, but I've checked several organizations inclusing a few already several years old but I can't find any reference of it.