Magus is a red T3 because it has a very big slant towards being played as a bruiser or blaster, in T4 fashion, except with wizarding tricks on top. His own spell list is a bit too limited (with a zealous focus on blasting and martial stunts, and a small smattering of invisibility, teleports, minor illusions, and walls), missing most of the skill stunts, divinations, crafting, and spells to handle social situations. He has number of solid 1st through 3rd level spells, but after that it starts to fall off, although the Spell Blending arcana will give the Magus access to choice Wizard spells (it uncomfortably penalizes you for trying to get access to Wizard spells of your highest spell level though) for regular T3 status. The main reason I regard it as low T3 is really because people tend to go very martial on the Magus and tend not to develop the breadth of solutions that typify T3, although even without that it is still T3, just on the lower end.
Warpriest I guess I'll move back to regular T3, although I generally feel that Bard and Inquisitor do much better and that it is a shitty Cleric minus on the whole. It has no discounted spells, no domain spells, shitty blessings instead of domain powers, and a slew of buffs in places it doesn't really need. There is honestly no good reason to play a Warpriest over a Cleric unless that reason is "I think Clerics are overpowered." There's little reason to play a Warpriest over an Inquisitor either. Still the class works fine and it does get a decent selection of spells for a variety of situations.
Mesmerist is T3 because it has a solid versatility in its abilities. It might not be as potent as a martial but it has a solid variety of problem-solving tools thanks to its divinations, illusion series of spells, enchantment series of spells, and it has access to many of the best early spells (grease, sleep, color spray, obscuring mist, glitterdust, invisibility, pyrotechnics, silence, blur), plus tricks, stares, and touch treatment. So basically it stays useful in a good variety of circumstances. Even against mindless enemies it can still use its stares (psychic inception stare power) and tricks to contribute as well as spells like grease, alter self, glitterdust/pyrotechnics/obscuring mist, defensive illusions, teleportations, solid fog, and shadow evocation/conjuration (mindless enemies tend to have shit will saves). If it's not PFS then Draconic Malice lets him hit anything living with mind-affecting fear spells and even without that he can use Psychic Inception to cast mind-affecting spells if desperate. The Mesmerist is also not brought down by psychic casting as much thanks to the fact that he has a solid bunch of spells that do not have an emotion component and he gets a bonus on will saves and he can remove shaken from himself as a swift action, giving him decent countermeasures against the usual weaknesses of psychics. Basically the Mesmerist isn't always strong (although he can do some very silly things with Mind Swap or trivialize an entire encounter with Confusion) but he does have the tools to consistently be a significant asset to the party, played right.
Looking over the Medium class, I'm listing a Medium that can't seance well as T4 and Mediums that can are kept as T3. I'd say that the Medium archetypes that do not have problems channeling spirits (Fiend Keeper, Uda Wendo, Relic Channeler, and Rivethun Spirit Channeler) are probably the best. The Medium does have the weaknesses of psychic classes and if you have trouble channeling spirits it nosedives hard. It's not even full BAB to compensate for its 4th level spellcasting.
Both Mesmerists and Mediums should probably invest in a Lesser Metamagic Rod of Logical Spell though.