What if you give the most underdog faction a 10% gold buff? This will make the new players and herp derpers go “OMG more gold!” and join the faction.
But it if finally gets balanced within 10% max pop on all 3 sides or so, EVERYONE gets +20% gold for being balanced factions.
This will lead to underdogs getting populated over time and everyone getting grumpy about not have balanced sides and trying to fix it actively for the gold bonus instead of just complaining about it being unbalanced.