Try
SELECT Amb_id, Location, IIf([amb1_time ]<=[amb2_time] And [amb1_time]<=[amb3_time],[amb1_time],IIf([amb2_time]<=[amb1_time] And [amb2_time]<=[uttr3],[amb2_time],IIf([amb3_time]<=[amb1_time] And [amb3_time]<=[amb2_time],[amb3_time]))) AS min_callout_time
FROM YourTableName;