What does it mean that three of the first were plucked up by the roots?
It means three of the ten nations that originate the European community will lose their identity and be absorbed by the other seven. These smaller nations are plucked up by the roots, leaving seven kingdoms from which the little horn, the Antichrist, will arise.
Les explains that the little horn comes out of the ten nations or kingdoms that make up the revived Roman Empire, pictured as the ten toes of Nebuchadnezzar's vision. Before this little horn, three of the first were plucked up by the roots. Les teaches this simply means that of the ten nations that originate the European community and their parliament and government, three of the smaller ones will lose their identity and be gobbled up by the other seven. Then out of the remaining seven will come the man Antichrist. The context also describes Europe's current turmoil and readiness, making the time ripe for a strong man.