You're confusing regular expressions with 'like' conditions.
Like checks for exactly the string you pass to it, with the only three exceptions being:
1. % - any number of any characters
2. _ - any single character
3. ESCAPE clause that lets you define escape character
so you can use % as 'I'm looking for percent sign'
[ABC] meaning 'either A, B or C' is a regular expression syntax.
You may want to use regexp_like instead, or for such simple query just 'OR' several likes:
with examples as (
select 'Adam' ex from dual union all
select 'Dorothy' ex from dual union all
select '[ABC]something' ex from dual union all
select '[ABC]' from dual
)
select e.ex,
case when e.ex like '[ABC]%' then 'MATCHES' else null end as matches_like,
case when regexp_like(e.ex, '^[ABC].?') then 'MATCHES' else null end as matches_regexp_like,
case when e.ex like 'A%' or e.ex like 'B%' or e.ex like 'C%' then 'MATCHES' else null end as matches_complex_like
from examples e