1/2 and 1/3 Casters and casting level.

Hi all.  Sorry if the answer should be obvious.   I’m new here and new to this edition of the game.   
my question for the group is, when a cantrip description says that a spell changes (eg does more damage) when the caster reaches a certain level does than mean the level of the caster regardless of whether they are full, 1/2, or 1/3 casters?

The example that brings this up is an Arcane Trickster casting True Strike.  The spell description says that the spell does 1d6 more damage when the caster reaches 5th level.  Does the AT get that bump at 5th level Rogue or as a 1/3 caster would he need to be 15th?

Quote from Aelphstan >>

Hi all.  Sorry if the answer should be obvious.   I’m new here and new to this edition of the game.   
my question for the group is, when a cantrip description says that a spell changes (eg does more damage) when the caster reaches a certain level does than mean the level of the caster regardless of whether they are full, 1/2, or 1/3 casters?

The example that brings this up is an Arcane Trickster casting True Strike.  The spell description says that the spell does 1d6 more damage when the caster reaches 5th level.  Does the AT get that bump at 5th level Rogue or as a 1/3 caster would he need to be 15th?

 Welcome on board! 

The damage increases for cantrips are based on your total character level. So, regardless of whether you are a full, half or third caster, whether single class or multi class, or even whether you obtained your cantrip from your species or an origin feat and are a single-classed Barbarian, your cantrip damage will increase at levels 5, 11 and 17.  
(e.g. A Sorcerer 2/Fighter 3, an Eldritch Knight level 5 or an Infernal Tiefling 5th level Barbarian casting Firebolt will all do 2d10 damage.)

I hope this helps. 

Just to expand on what ThelenyiWhinlaw said, you can find the rules that state this in chapter 2 of the Player's Handbook or the free Basic Rules, in the Spellcasting section under Multiclassing:

Cantrips. If a cantrip of yours increases in power at higher levels, the increase is based on your total character level, not your level in a particular class, unless the spell says otherwise.

While it says "unless the spell says otherwise", as of this writing there aren't any cantrips in official material that say otherwise. There could perhaps be one added someday, but for now they all operate based on total character level.

Quote from wagnarokkr >>

While it says "unless the spell says otherwise", as of this writing there aren't any cantrips in official material that say otherwise.

 I think at one point during playtest eldritch blast was based on warlock level.