Your 'Canon' Inquisitor you'll be taking to DA4 and why they're Canon for you.

The Inquisitor

Name: Maximilian Trevelyan
Race: Human
Class: Mage
Gender: Male
Romance: Josephine

Declared for: Restoring order, blah blah.
Herald of Andraste: Privately skeptical, publicly embraces role.

Plot missions:

IHW: Conscripted the Mages-Maximilian respects Fiona and sympathizes with the rebellion, but the Breach took priority at the moment, and conscription seemed more politically expedient. 
WEWH: Gaspard rules alone-Celene's incompetence and love of the Game started the entire mess in Orlais. Maximilian needed troops, not schemers.
HTLA: Wardens allied with the Inquisition, Hawke left in the Fade in favor of Loghain-Hawke was a known mage radical and apostate, much less useful than Loghain, who would help reorganize and stabilize the Wardens.
WPHW: Aided the Sentinels, respected the rituals, but Morrigan drank-Max was genuinely interested in the Temple and its history, but recognized a geass when he saw one.
Divine Victoria: Vivienne-The only freedom for mages will be institutional-Vivienne recognized this.

Inquisitor Ameridan: Revealed the truth about him being an Elven Mage-Further legitimizes mages in positions of power and hurts the image of the old Chantry.
Exalted Council: Inquisition downsized under the Chantry-Maximilian would never give up what he's built.
Fen'Harel: Attempting to redeem-Was friends with Solas, and thinks the situation can be contained.

Companion outcomes:

Blackwall: Warden-Too poetic to pass up.
Bull: Killed during Trespasser-Max didn't trust Bull (an admitted Qunari agent) or the Chargers (possible sleepers) for an instant. The Qunari alliance seemed suspect too, but they did send him useful intelligence. This didn't make up for what happened during Tresspasser, but the Venatori did less damage than they could have. 
Cassandra: Still with the Inquisition-Frankly, Max dislikes the idea of the Seekers, and his Inquisition has provided a more than adequate replacement anyway.
Cole: Spirit nature restored-Trying to make Cole fit the mold of a 'human' seemed irresponsible and presumptuous. Cole isn't a person, and shouldn't be forced to act like one.
Dorian: Convinced him to reconcile with his father.
Sera: Didn't get along very well, but she never left.
Solas: Helped rescue his Spirit friend, protected mages.
Varric: Discovered Bianca leaked Primeval thaig location.
Vivienne: Given the Snowy Wyvern Heart for Bastien.

Advisors

Cullen: Had him stay on lyrium-The middle of a war seemed like a bad time to try and get off of lyrium. If Cullen couldn't try again later, it's his own fault.
Josephine: Took the long way with removing the contract, as she preferred.
Leliana: Steeled-Spymasters need to be able to do whatever it takes.

i don't know i tried 3 playthroughs and didn't finish any of them, in fact i deleted them all, a very painful and dreadful game to play.

but some "key" decisions i guess will be:

mister moustache left in the fade.

Inquisition disbanded.

Sided and allied with Templars.

the best good ending from the celene quest (dunno cause i didn't played that part xD)

i have to decide if i want Leliana or Cassandra to be divines. (i'm not sure, cause my main canon warden has a romance with her and i don't know what is best even if i read the epilogue slides)

the rest, well as bioware erased the classic and cool import save, and now we have that shit called keep, i will fill the rest in there when the time comes.

for the question "why they're canon for you". Cause is the continuity of the previous two games, nothing special.