How can one reconcile the statement that public dawah began at the end of the third year of the prophethood with the statement that it began with 'Umar ibn al-Khattab's embracing Islam in the fifth or sixth year?

The Messenger of Allah, peace and blessings be upon him, spent three years secretly inviting people to Islam, until Allah commanded him to proclaim the دعوت by saying: ﴿So proclaim what you are commanded and turn away from the polytheists.﴾ As for the conversion of Umar ibn al-Khattab, may Allah be pleased with him, it occurred approximately two years after that, in the sixth year of prophethood, three days after Hamza, may Allah be pleased with him, embraced Islam. The Prophet, peace and blessings be upon him, prayed to his Lord to strengthen Islam through one of two men: Umar ibn al-Khattab or Abu Jahl. The one more beloved to Allah was Umar. Umar was named Al-Farooq (the Discerner) because with his conversion, Islam became manifest and the Muslims grew strong, to the extent that they went out to pray at the Kaaba in two rows, after they had been unable to do so.