Are employees employed on a contract lasting less than three months still denied certain rights?

No. It used to be the case that employees working under a contract lasting, or expected to last, for three months or less were denied the right to statutory sick pay, guarantee payments, pay while on medical suspension and statutory minimum notice to terminate their contract. However, with the repeal of the relevant provisions in the Social Security Contributions and Benefits Act 1992 and the Employment Rights Act 1996 by the Fixed-term Employees (Prevention of Less Favourable Treatment) Regulations 2002 (SI 2002/2034), since 1 October 2002 those rights have been available to all fixed-term employees.