Omega 3 supplementation is great for any age, so do not be worried about giving this to your 3 year old. We have been giving supplements like this to our daughter since she was 2 with no issues at all. In fact, if you look at the research, it is recommended for pre natal as well.
The one issue you have to take a look at is that omega 3's from fish oil are not absorbed into the body as easily as the form of supplement we use, which is from Krill oil. The reason is that fish oil omega 3's are suspended in triglycerides, which need to be broken down by the body. Where Krill oil is suspended in phospholipids, which is what your cells are made up of and makes absorption almost instantaneous.
If you want more information on all of this just send me a message!
BTW, your whole family should be taking Omega 3 supplements, it has an effect on a wide range of health issues!