I have a 16 cha lvl3 lock with only 2 spells per day. Lvl 1 and 2 combined. Shadow heart is fine with 4 seperate lvl 1 casts and 2 lvl 2 casts.
I made my astairion an arcane trickster gave him intellect ciclet to set int to 18. He still has 2 casts.
Am i missing something?

No; warlocks only have 2 lvl 2 spell slots at lvl 3; however, they recharge on short rest and not long rest.

Gotcha. But plus stat items dont seem to increase spell usages per day.( Arcane trickster at concern this time)

Ability points do not grant spell usage or slots

Ok. It used to grant in other dnd games....

In D&D 5e, only wizard get more spell splots from ability (Intellect)