Uller
Adventurer
Show them, don't tell them.
The players that you do have use their gold for role-playing reasons such as lands and title and temporal power, reward them in game with the benefits of that expenditure. Describe their Castle, describe the benefits that flow from having such power. Engineer scenarios were those PCs can get problems resolved through their henchmen, or their armies or simply by their word or reputations. Set up scenarios were they can march their armies into battle with other armies; scenarios that can't be resolved with a plus one sword.
If you do it right the other two will catch on quickly, and before you know it be saving money for Keeps and titles and henchmen of their own.
Most gamist players are competitive beasts. If one PC has a Tower, and Towers matter, the rest are gonna want a bigger Tower than that other guy.
Maybe it's a phallic thing. I dunno. It's certainly something that happens in the real world so why should your games be any different?
I've been playing with them since 3e came out. In fact our first game was 2e...I don't think I'm going to change them. I don't WANT to change them. They like playing the game as gamists. Oh...and one is a woman. So...that pretty much shoots down that part of your theory.
I do reward them with in-game benefits. One of my players is rebuilding his ancestral estate (Thundertree from Lost Mines). He'll become recognized as a noble and game the various benefits associated with that background and have a steady income. Our wizard player is taking on an apprentice. He'll gain a useful and trusted cohort that can perform errands for him and even help him during adventures. But the other two aren't interested in that and will never be interested in that and, frankly, I'm fine with that. I enjoy their contributions to the game and their company at the game table. I want them to have fun in the ways they want to have fun as long as it isn't interfering with the fun of the other players or myself.
The problem some people are having is not that they are unhappy with the playstyle of their players that expect in-game uses of in-game rewards to advance their goals in-game. The problem is that playstyle is not that well supported when it comes to the in-game reward of monetary treasure...particularly in the APs published by WotC (at least the ones I've played so far).
I'm not saying your suggestions aren't good. But they aren't really solving the problem described. In PotA, it is possible for the PCs to go from 1st to 13th+ level with no real downtime. Once you get into the temples and defeat the first prophet, there is little opportunity for downtime organic to the adventure. So you have PCs finding reasonably large hauls of treasure and not much to do with it until the adventure is complete. If the players know that as soon as the adventure is over their PCs are going to be retired so they can make new ones to start a new AP then they start to see treasure as pointless and they are correct and that makes the acquisition of treasure not much fun. Suggesting that I give them good and interesting uses of treasure for after the AP doesn't really solve that problem. No one is going to think: "I better go build a castle/establish a wizard academy/found a church to my god/do some serious networking with the nobility" while faced with an apocalyptic on the verge of summoning a primal elemental power into the world or while trying to deal with a gaggle of demon princes reeking havoc in the underdark or while trying to defeat Strahd to escape Barovia or trying to fend off the Giant version of Ragnarok...
Yes...at some point all campaigns come to an end and treasure collected at the end is going to go toward the PCs living happily ever after. My players are good with that. They enjoy their PCs becoming NPCs and meeting them later to see what has become of them.
But that's not the issue. It's a 9th level PC finding a 500gp gem and having nothing to do with it other than stash it away with his other 2000gp worth of coin and gems he has nothing to do with at the moment because he already has all the best gear he can buy and tomorrow he has to go explore the Fane of the Elemental Eye before one of these cults makes another devastation orb and takes out another town...My solution to this problem was to make a few roles on the various magic item tables and present to them a chance to purchase some of these using the Sane Magic Item Prices PDF that is out there. This somewhat satisfied them.
Now I am just going to let them exchange gp for xp. We don't get to play that much so I will place opportunities to find enough gold to gain a new level every 2-3 game sessions if they convert all their wealth to xp or every 4 if they don't convert any. To convert GP to XP all they have to do is use it for downtime activities. So that paladin building his family estate...he will gain 13th level a little faster than the warlock that wants to buy a useful magic item instead.