The answer to your question is on p.164 of the Player’s Handbook. Specifically, you are looking for this sentence.
You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. If you are a ranger 4/wizard 3, for example, you know three 1st-level ranger spells based on your levels in the ranger class. As 3rd-level wizard, you know three wizard cantrips, and your spellbook contains ten wizard spells, two of which (the two you gained when you reached 3rd level as a wizard) can be 2nd-level spells. If your Intelligence is 16, you can prepare six wizard spells from your spellbook.
So the answer to your question based on your example of Wizard 3 and Cleric 2 would be that you can known/prepare up to level 2 spells from Wizard and level 1 spells from Cleric.