My initial design was to aim for a system where a generic character with an even mix of those stats would be quite effective, and to make sure that maxing a single stat was not the optimal strategy. There should be reasons to have a massive single stat and builds that make it workable but I definitely don't want it to be the flat out best. If a class strategy guide can be summed up by "Just max your Strength" then I have done something terribly wrong. It is boring and it means that people are stuck choosing between being bad and adhering to a single build. Yuck.
Strength and Dexterity are relatively easy in this regard because you generally need both of them to be successful. If you are going to deal damage you need to land hits, and if you are going to hit you want to make it count when you do! Mathematically speaking each point of Strength increases the relative value of Dexterity (and it works the other way around too) so as long as the point at which they are roughly equal in efficacy is close to where the raw numbers are equal all is well.
Constitution, on the other hand, is a problem. When characters use regular Powers that bonk an enemy everything works because all three stats contribute to their effectiveness. The trouble comes up when I get creative with Powers and people don't necessarily need any Strength or Dexterity at all. Example:
Battle Orders: Rank 12
Choose 2 allies. Each of them makes a Basic Attack and gains 2 Focus if they hit.
The problem with Battle Orders is that a character with 3 Strength, 3 Dexterity, and 12 Constitution can spam Battle Orders all day and be amazingly effective. They don't deal damage personally but their friends absolutely rip through the enemies. This can be exacerbated by having allies that stack Strength and Dexterity rather than Constitution because they spend so much of their time beating down without using a Power at all. The optimal group is one where people's stats are all extreme and only a couple of Powers ever get used and that lack of diversity is terrible. A regular balanced group of 4 players would get demolished by a group running 2 people spamming Battle Orders and 2 people optimized for making all the Basic Attacks.
I have been spending time going through all the Powers and making sure that this sort of thing gets squashed. I want there to be interesting combinations, and there are, but I don't want a class to be reduced to a single Power with a single stat dominating all other strategies. There are penalties to such a build of course, like the fact that the person using it is probably completely helpless in a duel, but I don't much like using that as a balancing mechanism. Saying to the GM "Look, just make sure each individual player has to fight deadly enemies by themselves on a regular basis" seems like a terrible way to set up the system. The way I fix these issues is by making sure Powers at some point use characters' actual damage stats, like in the following new version of the Battle Orders Power:
Battle Orders: Rank 12
You and 1 ally each make a Basic Attack and gain 2 Focus if you hit.
This is a simple, small change but it fixes the problem completely. The Power is still good, especially if you have an ally who is set up to deliver deadly Basic Attacks, but if you try to run it with miserable Strength and Dexterity your attack will be almost irrelevant. On a balanced character this new version is pretty much the same, but on a single stat character it is reduced in effectiveness a good 40%, which puts it in line with other choices you could have made. The single stat character is functional but won't eclipse anyone else as long as the downsides of their build actually matter.
There are definitely some exceptions to this, notably Powers like this:
Defiance: Rank 10
Gain 13 Focus.
This Power, while it is spammable and doesn't use Strength or Dexterity, isn't leading to any broken builds or combinations. Sure, you can build a character who is really good at standing there and taking a beating while healing themselves, but so what? You aren't contributing anything to a team (being invincible but irrelevant is rarely useful) except when you can block a narrow corridor, I suppose. There are similar Powers that restore Focus to other people though, and those ones have the same issues that Battle Orders does. A little worse even, because then there will be tons of pressure to play the single stat healer build and somebody will have to suck it up and be the Cleric. I say faugh to that.
Here are some more examples of Powers that I have come up with that allow people to do cool things that aren't direct attacks necessarily but which make sure you can't abuse a single stat build.
Rejuvenate: Rank 6
Restore 6 Focus to an ally and on your next turn your Hit Rolls have advantage.
Sacrifice: Rank 10
Restore 7 Focus to an ally and if that ally takes damage from an enemy during the next round you deal +20 damage during your next turn.
Energize: Rank 10
Attack 1 creature for Lightning damage. If you hit, you or 1 ally restores 10 Focus.
Shared Power: Rank 12
One ally regains 8 Focus. On your next turn you deal +13 damage.
In each case the Powers have lots of non damaging utility but if you refuse to ever make attacks you simply aren't going to be very effective. I like the idea of classes that help defend and assist their allies but I really want to be sure that nobody has to play the healer / buffer build just to have a good group. If you want to single stat Constitution these powers you can, and you will be a contributor, but that strategy is only viable, not exceptionally powerful, and definitely not necessary.