Longmane,
This is a very appealing approach. Three stats is not overly complicated, yet it should be complex enough for the needs of a game. All of these three seem to be about appearances that are based on the choices an individual knight makes.
1) Honour, as you describe it here, would best be left with the players to decide. It is something quite intangible and hard to define exactly, with much room for debate. But it does sound very much like something that the noble society as a whole would know how to measure. You could give out "honor marks" (one / target character), where the ruler would have the most marks and a simple knight just a few. Foreign marks count for half, and marks received from your liege count as double.
2) Prowess, on the other hand, could be calculated by the game, and could gradually diminish over time. The game would keep a track of important events (winning a tournament or a duel, participating in a conquest of a city etc.) and have a base value calculated for that, which does not diminish (or does so very slowly). On top of that there would be the more fluctuating value calculated on more mundane battles and such. Add them together and we have your current prowess.
3) Largesse, however, is the trickiest of them all. Hosting a tournament and setting a decent prize for it would definitely count, as would family investments to a region - especially to one that is not where you have your estate. Or it could be doing civil or courtier work outside of your own region, if using your time could be included as well as using your gold. Maybe giving a unique item (in a good condition) to another noble could count as well (and once given out, the item would no longer give largesse bonuses). Still there is a strong feeling that an ordinary knight should be able to gain it as well, and I then need to again mention banquets and hunts held in his estate, where he can choose how much gold he spends on the arrangements.
Then there is the question of what these stats would actually accomplish. Right now honor and prestige become rather meaningless after a certain point, so I would really like to see all of them displayed in relative terms based on all the values in a realm (and when viewed individually, a comparison with your own stats). This would prevent all kinds of farming of a certain stat encouraged by the realm. If everyone displays much prowess, nobody stand out.
The effects of prowess might be the easiest to determine. Those with most prowess could recruit the largest number of men and also have access to all the types of units. Those with very little prowess (relatively speaking) can only dream of recruiting special forces and cavalry, and would not be able to have all that many archers or infantrymen either. Perhaps the very best archers and infantry units would be out of their reach as well. It's simple and intuitive.
Honour, on the other hand, could have a broader effect. Since it really is a statement about who you would like to succeed in his endeavors it could reflect that by giving a morale bonus to your unit and have your courtier activities be more efficient. Perhaps the peasants would be happier with a honorable lord as well, and all the peasants would be more loyal if the ruler of the realm was one of the most honorable nobles in the realm. Having a honorable general and marshal could have some positive effects, etc. This would encourage the characters to act in a way which would encourage the nobles with strong titles to value you more than the guy next to you.
Largesse is the true challenge here. If you spend your gold in ways not directly giving a benefit to the realm, the players will not appreciate that. Even if you manage to have a higher stat, it should not mean to the players that this is the one who wastes their gold and this is why they are losing the war. So whatever the activities are that gain you largesse, they should give real and tangible benefits to the others while nothing by itself to you.
The actual stat would tell the others what a nice guy you are, but what else would it give? Perhaps that by itself would be enough? High largesse might indirectly help you to get honor marks from the others. Or could it be that whatever you do to gain largesse gets more efficient with a high relative value? That in turn makes it a feedback loop, which is not nice. My mind is frozen here and I can think of nothing else but random benefits that you get here and there. "The reputation of your largesse gives you X."