select max(myDate) from

( select field_1 myDate from myTable

union all

select field_2 myDate from myTable

union all

select field_3 myDate from myTable) as Temp

It will return the maximum date of the three date columns values.

Example :

If u r data would be like below

select * from myTable

field_1field_2field_3

1/1/20102/2/20103/3/2010

1/1/20112/2/20113/3/2011

Query Returns :

2011-03-03