Multi Classing Help

I am new to multi classing and new to spells with 5e. Can someone help explain it to me. Currently I am a Level 3 Rogue (Arcane Trickster) and just leveled up to level 4. For my fourth level I was going to put a level into the wizard class as it makes sense for my backstory. My current spells are [Cantrips]: Firebolt, Mage Hand, and light. [Level 1]: sleep, find familiar, and color spray.

My Stats are Str: 11 Dex: 18 Con: 14 Int: 17 Wis: 14 Cha: 7.

Specifically The spell slots is what is confusing me. I am not sure about the calculation for them. According to the PBH I would take my wizard levels (1) and my rogue level / 3 rounded down (1) and add them together for a total of 2.

Then using the table on page 165 it would seem that I’d have 3 first level spell slots.

But that doesn’t make sense to me as I originally had 2 spell slots as an arcane trickster and a first level wizard has 2 1st level spell slots. So would I have 4 spell slots or only 3?

Lastly can I use my spell slots on any of the spells I have (no matter whether it is from the wizard class or the Rogue (Arcane Trickster))?