The RAP3 is a newer AP which may have firmware that wasn't supported in AMP 7.4.2.
To give you an idea, 7.4.2 was released in November 2011, and the RAP3 was released around July 2012 (this matches closer to AMP 7.5.5). You're options for RAP3 support in AMP are to either upgrade to the last release of 7.5 (7.5.7) which gets you the firmware support or to the latest release: 7.6.2 which may have some fixes. If you're using the RAP3 in an Aruba Instant state, then you'll want to aim for 7.6.2.
Some more notes:
7.6 was released in November 2012, with the latest patch (7.6.2) published last month (January 2013). Based on customer feedback, the release has been very stable.