The Pact Magic paragraph on page 164 of the Player's Handbook multiclassing section reads:
you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know.
If I were to multiclass as a Warlock 5 / Rogue 3 Arcane Trickster and cast a Warlock spell using my Arcane Trickster's 1st level spell slot, would it be cast as a 3rd level spell?
From the Warlock Spell Slots section on page 107 of the Player's Handbook:
The table also shows what the level of these slots is: all of your spell slots are the same level.
The proposed duplicate question appears to only address the issue of spell slot recovery, not casting.