Adding counts inside a COALESCE
I'm giving it a shot, still not sure if I got it:

select 100.0*(count(b.taba_id) + count(c.taba_id)) 
     / (( select count(1) from b) + (select count(1) from c)) 
from taba as a
left join tabb as b 
    on a.id = b.id 
left join tabc as c
    on a.id = c.id


Some thoughts, count will newer return null so coalesce is pointless. If the above is correct it is possible to use window function (as I think you have tried to do), to replace the total count's