
Here are the basic steps:
You will need an environment (SharePoint Server + SQL Server) for each farm. While a specific version of SQL can be used across versions, it is cleaner if you have a SQL server per farm. WSSv3 needs to be patched to at least SP2. I would recommend patching the 2010 farm to SP2, as well.
To go from WSSv3 to 2010, make sure WSSv3 is fully patched (needs to be at least at Service Pack 2). Run the pre-upgrade check and resolve any errors, if applicable, using stsadm -o preupgradecheck.
Backup and restore your content databases to 2010. Create a Web App and attach your CDBs. This will upgrade them.
For the remainder of the upgrade strategy, see the document at https://learn.microsoft.com/en-us/sharepoint/upgrade-and-update/upgrade-from-sharepoint2013-to-sharepointserver-2019.