How do I convert 3 metre Snellen chart readings to 6 metre equivalent

Very strictly, you can not. Someone modestly short-sighted would perform noticeably better on a 3m Snellen chart than a 6m one. The test distance can make a difference, and it is a piece of information worth preserving.

But in terms of simple angular acuity, treat it as equivalent fractions

''Three over six'' 3/6 => 6/12 (= 1/2, 0.5)