insert into A values (0, 'a', 'b');
is possible
insert into A values (1, 'b', 'a');
throws an exception
CHECK(a2 < a3)
only allows inserts where the value of a2 comes first in the alphabet (is lower), compared to a3.
You can try to execute select ascii('a');
so you can see the ASCII-code of the character (here: 97).
Edit: Be aware that capital letters like A has different ascii codes than small modes like a