hi han,
best thing to have is something fast acting - so lucozade, orange juice, jelly babies. it's recommended to have roughly 15g of carbs, wait 15 mins then test again to make sure you have come up. it's also recommended to have some slow-releasing carbs once your levels are above 4, eg a slice of toast or a biscuit (although some people choose not to - you'll work out what's best for you over time).
3am is the natural time to go low - i know that if i'm going to go low at any time during the night, it'll be between 3 and 4!