Is it normal to not lose anything... and then suddenly drop a pound a day?

Use measurements not the scale. You need to make sure your getting all the right vitamins / minerals your missing out on, supplement them. 3lb is alot so make sure you do this, and i assume you are on a 1,500 deficit then. Which is ALOT so expect to burn threw muscle too, unless your Body Fat (BF% x LB) x 31 = 1,500+.