1. Privately owned cannons: yes, and in fact, that's one of the things the British were dispatched to Concord on April 19th, 1775 to seize. 

2. Privateers (private sea-going vessels used for raiding and protecting sea commerce) owned cannons privately.

3. Most colonists fighting in the Revolution were using muskets, much as the British had. Many colonists owned rifles, but these were much slower to load than muskets, and often lacked fixture points for bayonets. Bayonets did a lot of the killing in infantry fighting in those days.