If Doris was 3 when she sadly died on 4th Sept 1935, she would have to have been born between 5th Sept 1931 and 4th Sept 1932.
But they had a son, John, registered in Jul/Aug/Sep 1931 the earliest he could have been born would be about end of May 1931 (being six weeks old by begining of July).
Nine months from then would be Feb 1932, thus narrowing the possible dates for Doris's birth.
If John was born later (Dawnsay - do you know your late father's birthday?) then the possible dates are even narrower.
There are a few possibities
a) We have not found the birth reg, as Jeffrey is spelt some unusual way on it - but this should be easier to check out as the range of possibe dates decreases
b) She was adopted - as has been suggested.
c) She was not actually 3 when she died - is the age clear on the cert or could it be a badly written 8, for example, or anything else? If, say, an 8, she could be Frederick's daughter from a previous marriage, of which we do not know the maiden name.
Just some thoughts