hibbeln,
I recommend cutting one day each out of Istanbul and Cappadocia, and inserting 2 days of Antalya.
If you do not need a day to rest/relax after traveling to Kayseri, then Cappadocia is very doable in 3 days. In fact, unless you are big on hiking/biking or medieval Byzantine cave churches, anything more than that may be too much.
Typically people stay in Goreme, Urgup or Uchisar (all within 5-10 km of each other). I would recommend Goreme the most. This is a 2 hour bus ride from Kayseri, and I am assuming you are heading that way, as it is generally the main attraction in Cappadocia.
I recommend doing the balloon rides, goreme open air museum and taking one or two hikes (walking path maps are usually available from any hotel), and take in the amazing panorama. Also experienced travelers can engage local people/guides to go to the more secluded historic sites which you can enjoy without the throngs of tourist that overwhelm the popular sites.
The main sites in Istanbul can be done in 3 days. The top 3 or 4 of any traveler is usually Hagia Sophia, Blue Mosque, Topkapi palace, and Covered Bazaar. The first 3 are walking distance from each other, and the 4th is a 10 minute tram ride from the first. So if you really want to you could do all those in 1 (long) day.
As you are travelling in March, it is too early to be on the beaches in Antalya, howerver hiking should be quite nice. If you like hiking, I recommend taking a trip to Termessos (
http://en.wikipedia.org/wiki/Termessos), an incredibly well preserved ancient city in the Taurus mountains, about 30 km north of Antalya. In addition to that, Antalya boasts a nearly endless choice of historical sites.