It means your Daimyo has 3 extra force, on top of whatever force it has already.
There's another card called "Path of the lion": "Your Daimyo has +1 force". If this meant your Daimyo was now 1 force "total", the card would do literally nothing. I believe it's worded this way so that the cards can stack. By getting both of these cards, your Daimyo would be worth a total of 5 force.
I believe this bonus also applies to any monster that counts as a Daimyo. So, you could eventually make the Yurei into a 6 force monster. That's just my interpretation of the rules, though.
All correct as far as I can see.