From what I've understood you get the highest spell slot level you have access to to unlock known spells. Currently in 5e you can end up having higher spell slot levels than known spells (combining half casters with casters for an example, since spell slots are calculated based on a mixed progression). But I could be wrong.
In 5e, Spell Slots are determined by your Total Caster Level, which you calculate by adding 1 from full casters, 1/2 from half casters, etc. Warlock is its own thing. A Wizard 3/Cleric 3 has the same # of and level of spell slots as a Wizard 6.
Spells Known & Prepared are determined by your individual classes. A level 3 wizard/X "other" is effectively just considered a pure level 3 wizard when determining how many wizard spells they know and can prepare each day. But they can upcast their wizard spells into higher level spell slots (if they have any).