What level are spells cast at if a warlock multiclasses as another caster?

Question

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.

Answer 1

It depends on which slot you use.

If you cast using a warlock's Pact Magic slot, the spell is cast at the level of the warlock slot.

However, if you cast using your other class's Spellcasting slot, then it will be at the slot level you choose to burn.

You can use either class to cast the spell provided that the spell is low enough level to fit in the slot used (you can't cast a 6th level spell in a 3rd level warlock slot).

To answer your specific example:

You can cast your arcane trickster spells with your warlock Pact Magic slots, they would be cast at 3rd level. However, if you try to cast a warlock spell using your arcane trickster Spellcasting slots, they will be cast at the level of the slot you use. Also, you can't cast a higher level spell than slot you have remaining (can't cast a 3rd level warlock spell in a 1st level AT slot).