It's 2024 rules, that's why I was asking. The way it reads seems to imply you can prepare spells for slots you have for each class, and you have 3rd level slots.
You don't prepare
for slots.
The 2024 text says "You determine what spells you can prepare for each class individually, as if you were a single-classed member of that class. If you are a level 4 Ranger / level 3 Sorcerer, for example, you can prepare five level 1 Ranger spells, and you can prepare six Sorcerer spells of level 1 or 2 (as well as four Sorcerer cantrips)."
The key text is "as if you were a single-classed member of that class". Left out the explicit text saying "of the level you have achieved in that class", but it is picked up with the example, and reinforced again in the Spell Slots section.
So, if you are a Rogue/Arcane Trickster 9, you prepare spells as if that were ALL you were. Then you repeat the process as a Wizard 3. Now you have a list of prepared spells, each with its own spellcasting stat (if they are different).
In play, you expend slots to power those spells, and you have the multiclassing table to determine your slots.
For this character it is treated as level 8 - 5 levels from rogue (half, rounded up), and 3 levels from Wizard.
And yes, it is awfully phrased. (At least, unlike the hiding rules, the examples make the intention clear).
Cheers!