Three units are plenty. Just take larger squads. There are other units you can take to make a themed list, like Vipers and shinning spears and such. .
You could do that, but I think the ideal solution would be to go back to having subfactions that could treat certain units as troops, like Saim Hannah jet bikes (which eventually became troops for all elder before moving back to fast attack), Iyanden wraithguard, and Deathwing terminators. This is especially the case where the CP disparity between battalion and non-batallion lists has become so pronounced.
I'm not too sure it's a good idea, it would be really hard to balance out and make it work.
Supposedly you want to apply this kind of rules, taking Eldar as an exemple as it's the only army I know. Sam Hann would get scatter bike with 3 of them as troops and take a load of them. What would you do for ulthwe then, give them warlock council as troops? Boost there guardians? And Bieltan, which is a aspect type of army, would you give elite units the troops treatments? It's realisticly not feasible, fluff yes but feasible no.
One option could be having flexible organizations slot for a kind lower efficienty bataillons (3cp instead of the new five for exemple). And that one would be fitted with the special rules of a craft world chapter tactics or what not.
An other option could be having a warlord trait that unlock said unit stacking. You are trading one benefit for an other. Seeing the balance of the game right now I find it hard to think of other options.
You can't give any army free benefit. It won't work with the current player base, a trade off is nessecary.