By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
with kk as (
select 1 Id, 10 as one, 30 as two, 20 as three union all
select 2 Id, 50 as one, 60 as two, 20 as three
)
select
Id,
(ifnull(one,0) + ifnull(two,0) + ifnull(three,0))/
(
(case when one is not null then 1 else 0 end) +
(case when two is not null then 1 else 0 end) +
(case when three is not null then 1 else 0 end)
) as average
from kk
| Id | average |
|---|---|
| 1 | 20.0000 |
| 2 | 43.3333 |