Yep, 3 was a good number for me.
I fully understand the need to treat players who have concussion, but why should that be granted a free substitution? Doesn't happen for any other injury. I know the idea is that it prevents managers just suggesting he can run if off, but if the physios can see the player is concussed, they should be removed just like any other injured player. If you've made all your subs and lose a player to injury, tough luck for having played your full hand of subs and not keeping one back!
Also, as suggested previously, it's possible to feign the symptoms of a concussion to gain that extra sub. You'd like to think it wouldn't happen, but remember "blood-gate" in the rugby, where your man bit into a blood capsule to make it look like he was bleeding and gain that extra substitution!
Perhaps a neutral official physio could play the part of a 5th official, purely to assess a player's ability to stay on, as opposed to relying on the word of that club's own staff (who could hypothetically be in on the act)? Probably becomes a bit much for what would be a relatively rare requirement though...