How to Rend Fiends and Immolate People: A Guide to Sorcery

[Coffee_Dragon]

For example, if I Twin a Ray of Sickness spell (which is 1st level) and cast it using a 3rd level slot, does that require 1 Sorcery Point or 3 Sorcery Points?

The latter. PHB 201: "When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting."