License keys for SB 2.x doesn't work in SB 3.x. This is a right behavior. If you performed an upgrade from 2.x to 3.x the key must be also upgraded automatically. If not, so you already made a step in the right direction and contacted the support. Please wait for reply from them. They'll resolve your problem.
Guys, Sitebuilder license key for v.2.x will not work on v.3.x and it must be upgraded before uploading. Here https://register.swsoft.com/sitebuilder_key_upgrade/
it is possible automatically upgrade your license to v.3. Before upgrade please read all compliance issues that can prevent a license key from auto-updating.