1) The DLC fits in well in the order released. Ending --> Jaws of Hakkon --> The Descent -----> Trespasser (this one locks out everything else, FYI; its the 'epilogue')
Since Jaws of Hakkon is an 'open zone', you can feel free to return to go more completionist on it after Descent.
2) If you want to do them (not counting Trespasser) before the ending, you can. Being well into the Lv 10s is suggested (technically early 10s is possible, but IMO too hard to be worth it, nor really fitting in at that point in the main plot), and Lv 20s is best. Like Mass Effect 3, there's some 'gaps' in main plot where it makes a sort of sense that the Inquisitor may be up to 'other stuff' before the final fights/areas, so in there you can fit one or both of the DLCs.
There's a bit of different dialogue depending on if you do them during or post main game, but IMO not enough to make your decision hinge on it.
3) A good reference IMO is the (crafted) gear tiers for the content.
Tier 1 - Your time at Haven, ~Lv 5-10
Tier 2 - Your early time at Skyhold, ~Lv 10-15
Tier 3 - Your late time at Skyhold, ~Lv 15-20
Then only Tier 4 materials, not any schematics, are available in the main DAI game.
Tier 4 - DLC, all three. ~Lv 20-25(27?)
Its up to you if you want access to this Tier 4 Lv 20s stuff during a game that only presumes you have Tier 3 Lv 10s-early 20s available. I recall even the best loot drops in main game DAI to be Lv ~23, while the actual max character level is Lv 27.
All the DLC gives enough leniency to not assume you've completed any other DLC, though I'd suppose Trespasser (especially its higher difficulties) less than the others, in some ways. But of course doing all three, even just their main content, makes you a powerhouse by the final-final boss (Trespasser).
Basically, DAI may have a level cap of 27 but its Lv 20s content is relatively sparse compared to its Lv 10s, so DLC fills that out. Progression pacing-wise, that doesn't make it fit the best during the main game. They are also shown as mostly departures from the events of the main game, but also just enough of a side-adventure that they also make enough sense as an additional thing the Inquisitor can be up to.
Personally, my answer is no, don't do it before main quest. But its up to you.