What does it mean: "... was three days dying"?

Solution 1:

This is a somewhat archaic construction. The more modern way to say the same thing would be

It took the prince three days to die.

In my mind, the "was [timespan] dying" construct does serve to emphasize the fact that the person was dying the whole time, rather than "working up to it" in a sense.

(And just to be perfectly clear, yes, it does mean that the prince is now dead.)

Solution 2:

My view is someone that received a mortal wound and took three days to die. It gives me the feel that the speaker is "Southern". I don't think a "Yankee" would use the phrase.

Solution 3:

Based on what you've provided, it sounds like the scenario was:

• They lit the man on fire.
• Due to this, he was injured in some major way (e.g. internally).
• Over the course of 3 days, he was slowly dying from these injuries. Most likely getting worse after each day.
• After 3 days time, the man died.

It may be worth noting for clarity that this does not mean that the man was still on fire during the entire event, but that he continued to live through these damages for those 3 days.