Can a 12 year old watch a 10 year old and a 3 year old?

It depends on the kids ... mostly.

I did - but that was 60 years ago. (yes, we're still on speaking terms with each other). If you mean for long periods of time, then no. Legally, a child has to be at least 12 years old just be left home alone. A twelve year old is not emotionally mature enough to be responsible for the amount of care, time, and attention a 4 year old requires. And it would not be fair to the 12 year old to place such a huge responsibility on him. But if you need him to watch your 4 year old for a short while so you can tend to some household chores at home, then that should be fine - as long as you, or another adult is there.