3Shape is required yearly.
Edit: 3Shape made changes !!
Exocad is not required. Paying yearly which is not a license will give you new Exocad updates and support, stop paying and lose support and no new updates. Usually 2 new updates per year. Yearly cost is based on the modules you buy, separate fee for each module..
As an example, should you skip a month or 1 year of paying for support you must pay for the months you skipped.
Exocad changed the way you buy Exocad so best to talk to a reseller and go to the Exocad website.