Very odd. I watched a film last night called The Testing. It's set in the not too distant future where the nations of the world have agreed that there are too many people and each nation has a way of getting rid of 5% of it's population a year, some by culling older people, others by forbidding more than 1 child.
This film's set in the USA, where each year the kids are tested, from around age 6-7, and the lowest 5% are taken off for The Testing - put down in other words.
I do not believe that you can look at a person at such a young age and tell whether they will be a financial burden on society, or even a health one.
As adults we see people turn their lives around all the time, health-wise and income-wise, we see people do things they never thought they would do because of what's going on around them and inside them. We are influenced every day by what happens in our lives, in our world, in our society. We see good men commit acts of evil, successful, powerful men, and ordinary people with little in the name of wealth or health commit acts of heroism. We see children from dreadful circumstances, awful homes who would be written off at an early age, make something of themselves and others from 'good' homes do terrible things.
Nobody will every be able to convince me you could predict any of these things from the age of 3, because everything that happens to a person every single day, everything they experience affects their lives going forward from that point.
ETA: To add, from personal experience - my daughter, at the age of 3, was told she would never walk. She walks. At 8 that she would never be able to use a toilet, she does, at 14 that she would never be able to read - she's starting to (very slowly). If a severely disabled child can change so much against all the predictions, an able person would surely do many different things than expected from them at 3.