Prebid project maintainers have scheduled release 3.0 for november 2019.   Most important to remember:

-sizes are defined in other json property.

Old sizes definition:

sizes: [[300, 250], [300, 600]],

Prebid 3.0 sizes requirement:

mediaTypes: {
banner: {
sizes: [[300, 250], [300, 600]] }

-many adapters are not updated yet. All adapters that are not updated will be disabled. Just make sure that your adapter is on 3.0 compatibility list.