It depends on which slot you use.

If you cast using a warlock slot, the spell is cast at the level of the warlock slot.

However, if you cast using your other class's 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 slots, they would be cast at 3rd level. However, if you try to cast a warlock spell using your arcane trickster 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).

What level are spells cast at if a warlock multiclasses as another caster?
OK, I've waited until I finished ADWD in case there was any explanation there.

Why was Benjen Stark on the Wall? Was there an explanation I have missed?

I appreciate, as a younger brother, he would have had less opportunity than Brandon (or Eddard), and may have been looking for adventure or honour, but the Wall, by the time he would have joined, was hardly at the height of its renown.

Joining the Black wouldn't be, I would have thought, a particularly obvious option for Benjen, if he was simply looking for a title or a role in life to fit the name of Stark.
