update a 
set a.StartDate = c.StartDate, a.FinishDate = c.FinishDate
from TabA a inner join tabC c on c.code = a.code

update b
set b.price = c.price
from TabB b 
inner join TabA a on a.id = b.id
inner join tabC c on c.code = a.code

declare @t table (id int)

-- 1
update b
set 
  b.PriceFinishDate = dateadd(day,-1,c.PriceStartDate)

output inserted.id into @t(id)

from TabB b 
inner join TabA a on a.id = b.id
inner join tabC c on c.code = a.code
where b.PriceFinishDate is null and b.price <> c.price

-- 2
insert into TabB
  (id, price, PriceStartDate, PriceFinishDate)
select
  a.id
  ,c.price
  ,c.PriceStartDate
  ,/* ??? c.PriceFinishDate */ null
from TabA a inner join tabC c on c.code = a.code
inner join @t t on t.id = a.id

-- 3
update a 
set a.StartDate = c.StartDate, a.FinishDate = c.FinishDate
from TabA a inner join tabC c on c.code = a.code
inner join @t t on t.id = a.id