Many 3rd Hive topics Ive seen do balance quite well for NS. The problem is, the Devs want combat to be balanced as well. So thus, the 3rd-Hive-Ability-Balance-While-Keeping-Combat-In-Mind idea.
Xenocide________________________________________________________
Changes: Do double damage to structures. Should also be affected by silence, and according to the bug tracker, it should next update. This is the best 3rd Hive ability IMO, so thats all it needs.
Why this won't unbalance CO: Not many buildings aside from CC. 1 dedicated welder could potentially stop a whole team of xenociding skulks.
Web____________________________________________________________
Changes: The limit is way too low. I understand this is to reduce server load, but sheesh. You cant put up more than 10 or so webs without running out. If you place a new web, the oldest one should disolve if you are over the limit. NS maps are too large to be limited to such a small number.
Why this won't unbalance CO: Webs cost 3 points to get, and Bile Bomb is mostly useless since a Gorge attacking the CC wont last long. GLs and Welders are more common in CO.
Primal Scream____________________________________________________
Changes: IMO, the worst Hive 3 Ability. Nobody EVER gets this in CO. Triple the duration. Double the area of effect. I'd also like a small healing effect, so the Lerk would be able to fit the role he was designed for - support! Would need tested.
Why this won't unbalance CO: Gorges too slow/fragile to be a dedicated healer in an assault of Marine Start. For spending 2 points, would be nice to have an ability that helps you and doesn't suck. As of now, the duration is so low you pretty much break even on energy usage.
Acid Rocket______________________________________________________
Changes: Keep the current damage and energy usage as is. However, add an acidic effect. Meaning, after being hit, the acid persists on you and damages you over a period of time.
Why this won't unbalance CO: Resupply, which most Marines have, counters this.
Charge_________________________________________________________
Changes: Increase the damage a bit. Do double damage to structures. Should toss Marines around. Lower energy cost slightly. Would be nice if you could toggle on/off.
Why this won't unbalance CO: Not many structures. Toggle lets people use it as a speed boost more effectively, which is basically all it is used for. It better be good, since if an Onos gets it in CO, he only has 1 upgrade to choose from.