100 is written cent. It takes an s when it is not in the middle of a number.

Numbers Spelling Audio. Press to listen!
100 cent
200 deux cents
300 trois cents
400 quatre cents
500 cinq cents
600 six cents
700 sept cents
800 huit cents
900 neuf cents

No "s" here:

Numbers Spelling
103 cent trois
240 deux cent quarante
561 cinq cent soixante-et-un
796 sept cent quatre-vingt-seize