Bonne chance

3

because bonne is feminine and bon is masculine or it can be written in plural forms (bons, bonnes)

1

you write 'bonne chance' because 'chance' is feminine (la chance). When paired up with other nouns adjectives in french change 'gender' according and as Dustin Ryan has said, 'bon' can be written as 'bon' - masculine, 'bonne'- feminine, bons - masculine plural and 'bonnes' - feminine plural