How to store subset of columns from a csv file?
Hive's RegexSerDe which is often used to process logs could be used in your use case. You could use the regex to extract the first 3 columns from a line

CREATE EXTERNAL TABLE file (A STRING, B STRING, C STRING)
AS
ROW FORMAT SERDE 'org.apache.hadoop.hive.serde2.RegexSerDe' 
    WITH SERDEPROPERTIES 
      ('input.regex'='^(\\w+)\\t(\\w+)\\t(\\w+)(.*)')
LOCATION 'input/loading/'


The regex included is using pattern groups (\w+) to extract the first 3 columns and the last group (.*) is anything else which may be in the line

See more details here Community Article and Official Documentation