The biggest change in 3.0 is the requirement for PHP 8. There are some minor type-hint related changes, however the main APIs are the same as 2.X.
In addition note the following:
somnambulist/domain
has been updated and is now namespacedSomnambulist\Components\Domain
somnambulist/collection
has been updated and is now namespacedSomnambulist\Components\Collection