Running OpenHAB 3 as my main system and OpenHAB 2 with the MySensors binding, under docker, works great. I use the remote binding in OpenHAB 3 to connect to OpenHAB 2. Just make sure to run them on different ports if they run on the same server.

@TimO Thank you! It works for me with my Ethernet Gateway. I don't have that many nodes but the ones that I have, works as good as they did with OpenHAB2. I have a watermeter, an actuator and a door sensor. Great work!

The 2.5-version of the binding does not work out of the box in OpenHAB 3 for me.

2020-12-22 22:53:34.555 [WARN ] [org.apache.felix.fileinstall ] - Error while starting bundle: file:/usr/share/openhab/addons/org.openhab.binding.mysensors-2.5.0-SNAPSHOT.jar
org.osgi.framework.BundleException: Could not resolve module: org.openhab.binding.mysensors [272]
Unresolved requirement: Import-Package: org.eclipse.smarthome.config.core

    at org.eclipse.osgi.container.Module.start(Module.java:444) ~[org.eclipse.osgi-3.12.100.jar:?]
    at org.eclipse.osgi.internal.framework.EquinoxBundle.start(EquinoxBundle.java:383) ~[org.eclipse.osgi-3.12.100.jar:?]
    at org.apache.felix.fileinstall.internal.DirectoryWatcher.startBundle(DirectoryWatcher.java:1260) [bundleFile:3.6.4]
    at org.apache.felix.fileinstall.internal.DirectoryWatcher.startBundles(DirectoryWatcher.java:1233) [bundleFile:3.6.4]
    at org.apache.felix.fileinstall.internal.DirectoryWatcher.doProcess(DirectoryWatcher.java:520) [bundleFile:3.6.4]
    at org.apache.felix.fileinstall.internal.DirectoryWatcher.process(DirectoryWatcher.java:365) [bundleFile:3.6.4]
    at org.apache.felix.fileinstall.internal.DirectoryWatcher.run(DirectoryWatcher.java:316) [bundleFile:3.6.4]

I guess it might have something to do with this?

https://community.openhab.org/t/guide-binding-development-changes-for-openhab-3-from-2-5-x/104134

Namespace change of openHAB core classes
With the migration of Eclipse Smart Home to openHAB the namespace of all classes in openHAB core needed to be changed. This means all imports done by bindings that refer to org.eclipse.smarthome have been changed to org.openhab.core. With your binding code you can simply do a file level replace of this text string in the java source files to migrate the binding. On linux you can use the following commands in you src/main/java directory:

find . -name *.java -exec sed -i "s/org.eclipse.smarthome.core/org.openhab.core/g" {} ;
find . -name *.java -exec sed -i "s/org.eclipse.smarthome/org.openhab.core/g" {} ;