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.
You cast the spell at the level of the slot you use. If you use a warlock slot it is at the warlock slot level, if you use a wizard/sorcerer slot it is at the level of that slot.
If you have both the spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the spellcasting class feature, and you can use the spell slots you gain from the spellcasting class feature to cast warlock spells you know.
No you would get 2 1st level spell casting slots
So lets say you are a 5th level Warlock/1st level Sorcerer
You would get  3rd level spell slots from Warlock and  1st level spell slots from Sorcerer(or any other primary spell casting class). As you go up check the Chart on Page 165 PHB for spell casting multi class equal to your sorcerer level(because Sorcerer is your only spell casting class) to determine your new spell slot totals.
If you wanted to you could cast your Pact Magic to cast a 3rd level Magic Missile(from your sorcerer prepared spells) and attack with 5 magic missiles or you could use your spell casting and cast a 1st level Magic Missile for 3 missiles, it's up to you.
The other way around, you could use a 1st level Sorcerer slot to cast your Hex Warlock Spell. Or you could use your Pact Magic to cast the Hex spell at 3rd level(I think you get a longer duration at higher levels).
You have a misconception here.
The way warlocks gain spell slots is different from the other classes with the spellcasting feature.
Warlocks have pact magic, and their spells slots are always of the same level as in the warlock progression table. There are no "lower" spell slots, so people say they are always the maximum, but this is not accurate: they are always of the level shown in the table. Also worth noticing that pact magic slots recover on a short rest.
The other classes all have spellcasting. They gain spell slots of several different levels, the class levels stack (so a wiz 5 / cle 5 has spell slots as a 10th level), and they recover on a long rest.