The answer with artificial sweeteners is depends who you ask and how strict your fasts are. Some folks keep themselves to strict water only, others still see success with any number of substances or small amount of calories. It’s more personal preference than strictly physiological especially if weight loss is the primary focus.
‘Breaking a fast’ wouldn’t cause you to put on weight though. Going up 3 lbs could be any number of things from diet slipping or other normal weight fluctuations. Unless you ate an extra 10,000 calories last week, then it’s one of the later.