At a small scale, there are no costs involved. But be aware that the number of Scratch Orgs you can have varies with the edition of the org that you install the Dev Hub in:
Edition Active Scratch Org Allocation Daily Scratch Org Allocation
Developer Edition or trial 3 6
Enterprise Edition 40 80
Unlimited Edition 100 200
Performance Edition 100 200
If you already have an Enterprise Edition org you are all set; if not you should consider purchasing one.
Do consider how much the time of your three people is worth. Being able to share your work easily, and being protected from accidental overwrites via version control will in the medium term save you money; subscriptions to e.g. GitHub are not expensive. Unless the three of you are each working on entirely unrelated things, not having version control to synchronize your changes would result in a lot of manual copying.
Unless this is a very short "one-off" project, I would recommend following the documented processes rather than trying to roll your own. The newer Git, SFDX, Scratch Org, VSCode approach is a way more productive (and enjoyable) way to work than using the older tools. And the skills you pick up along the way will have lasting value.