Can a 3rd level sorcerer use a wand of a 5th level spell? I can't find this referenced in either the options for Use Magic Device nor Spellcraft nor the wand info in the CRB. If it's not there can it be assumed to not be possible? Thanks!
The only requirement is that the spell is on your class list. Your ability to cast it at the time is irrelevant.
The 3rd-level sorcerer absolutely can use wands of 4th-level* (sorcerer) spells.
Spell Trigger: Spell trigger activation is similar to spell completion, but it's even simpler. No gestures or spell finishing is needed, just a special knowledge of spellcasting that an appropriate character would know, and a single word that must be spoken. Spell trigger items can be used by anyone whose class can cast the corresponding spell. This is the case even for a character who can't actually cast spells, such as a 3rd-level paladin. The user must still determine what spell is stored in the item before she can activate it. Activating a spell trigger item is a standard action and does not provoke attacks of opportunity.
If a paladin can use a wand of paladin spells even if they can't cast any spells at all yet, then a sorcerer can certainly use a wand of a 4th-level* spell regardless of her spellcasting ability.
*Note that I say "4th-level spell" because 5th-level spells normally cannot be placed in wands.