If I multiclass as a warlock and another class, and I get spell slots from that other class, do they count as my highest-level slot or not? [duplicate]

Since warlocks have so few spell slots, they always cast with the highest level spell slot possible. So, if I got spell slots by multiclassing as a sorcerer (or wizard) and I cast a warlock spell using one of those sorcerer (or wizard) slots, would I cast it as the highest level I have (level 3), or would I only be able to cast a spell that slot level, but it would act as if I had used a level 3 spell slot or would I only be able to cast the level the slot is and that’s all? I know this is probably a stupid question but it might just work.