Three, not ten. The limit is the exercise, ten forces you to pad, and padding teaches your mind that the list is a chore. Three makes you choose, and choosing makes you look back over your day properly to see what actually stood out.
Write what happened, plainly, and then the part most people skip, why it happened, and what you had to do with it. The coffee was good because you got up before the others and had ten quiet minutes. Your friend called because you sent the message last week that made calling easy. This turns a list of pleasant accidents into a record of things you can partly cause again.
Some days you will scrape. That is normal and it is not a failure of the day, a small true thing beats a large invented one, and the bus being on time is allowed. It is on the thin days that the exercise does the most work, because it makes you search a day you had already written off, and you will usually find it was not empty, only unexamined.