There is a (standard SQL) syntax that is similar to what you propose but as far as I know, only Postgres has implemented it:
UPDATE users
SET  (field1, field2, field3)
   = ('value1', 'value2', 'value3') 
WHERE some_condition ;
Tested (for the infidels) in: SQL-Fiddle 
This also works in Postgres:
UPDATE users AS u
SET 
   (field1, field2, field3)
 = (f1, f2, f3)
FROM
  ( VALUES ('value1', 'value2', 'value3')
  ) AS  x (f1, f2, f3)
WHERE condition ;
This works in Postgres and SQL-Server:
UPDATE users 
SET 
   field1 = f1, field2 = f2, field3 = f3
FROM
  ( VALUES ('value1', 'value2', 'value3')
  ) AS  x (f1, f2, f3)
WHERE condition ;
and as @JackDouglas commented, this works in Oracle:
UPDATE users
SET  (field1, field2, field3)
   = ( SELECT 'value1', 'value2', 'value3' FROM dual ) 
WHERE condition ;