Protoss
* Photon cannon building armour increased from 0 to 1
* Photon cannon ground and air attack damage increased from 20 to 21
* Plasma shield upgrade cost decreased from 200/300/400 to 150/250/350
Zerg
* Dark swarm duration decreased from 37.8 seconds to 25 seconds
Terran
* Siege Tank range decreased from 12 to 11
Justification:
PvZ
From my observation, macro games are relatively balanced but protoss has too much trouble surviving various aggressive plays between zergling runbys, hydra busts, and muta/scourge attacks early game.
Photon cannons feel like paper, giving them more armour makes them slightly stronger against hydras, lings, mutas, and increasing their damage to 21 means they 4 shot hydras (25% faster kill time) and 6 shot mutas (17% faster kill time), and 2 shot 3/3 adrenalings (they can survive 3 hits?!) while not changing how fast they kill marines and vultures.
This should make relatively little impact on mid and late game but make the early zerg attacks much, much easier to defend. The +1 carapace upgrades will negate the difference later. +1 Building armour upgrade will help later, but it's probably negligible compared to how much it will help against early rushes.
Dark swarm duration shouldn't impact the PvZ matchup much; dark swarm is good but Zerg generally doesn't camp under for it for a very long time, and if he does it's pretty oppressive.
PvT
It seems that late game upgraded terran is a little hard for protoss to deal with, and reducing siege tank range should reduce the overall strength of a terran army if he spreads out, while Psionic storm can punish a more clumped army.
Siege tanks hitting nexuses from across the map in another base is also a little bit ridiculous.
ZvT
Zerg needs defilers to be able to survive against Bio terran, barely staying alive with a huge supply disadvantage in the most standard games, but the top zergs can seem unbeatable with their dark swarm defense, decreasing the duration gives more windows to punish zerg with for example sieged tanks which can pick off units hiding under dark swarm when it expires, unless Zerg is constantly baby sitting with replacing new swarms which can be killed by irradiate.
Siege tank range reduction might help slightly counterbalance swarm expiring much faster. Mid game pushes rarely leverage the full 12 range anyway