Eventually this 3rd slot might be added at high age like Info age or even Drone. But yes at some point, it becomes impossible catching up with 3 or 4 level 110 generals + planes ( 6 upgrades per age per plane... ) + Troops for TC or other donation like MRL / AC / MG / Infantry + Troops for one combo like HT Zooks + Oh no the meta changed so now i have to upgrade Mortars howies and APC + Oh no defenders did adapt so now i need AH too + Oh no i need munitions on tank depot so now i need to upate barracks tanks
Edit : The seasonal boost to encourage using new troops are great and will offer a better experience if we have 3 slots rather than 2