I think these are
the digits 0-9.
The first group
consists of digits 147, made out of straight lines.
The second group
consists of digits 2359, made out of some straight lines and some curves. (There are different ways of forming the digits, and some ways of writing 3 and 9 are all-curves.)
The third group
consists of digits 068, made out of curves only. (Some ways of writing 6 are a mixture of curves and straight lines.)
I am a little uneasy
about the asymmetry between 6 and 9 here, though I think it's fairly common to handwrite a 9 as a circle with a vertical line descending from it, and much less common to do an equivalent thing with 6. There's some ambiguity about 3, too.