So how did you get on?
Common error: many people start rounding from the last digit. 3·019451 does round to 3·0195 (4 d.p.) but NEVER round this up to 3·020! Always count along to the next d.p. past where you want to round to.
3 d.p. is generally accepted as being the right level of approximation for most answers. But there is another way of looking at approximation that you may have heard of: significant figures (s.f.).
In many ways it's more useful than d.p. as its basis is in actual measurement, rather than assumed perfection.