Etchmiadzin is the mother church and something like the “Vatican” of the Armenian Apostolic Church. The cathedral is considered to be the oldest cathedral of the world. The original church was build between 301 and 303 on the fundaments of an old temple and marks therefore the transition from paganism to Christianity. The core of the current building was built in 483.
Zvartnots
Zvartnots is a ruin of a 7th century cathedral which was dedicated to Saint Gregory. It had once a hight of 45m and was 37m large. It was found in the beginning of the 20th century and is one of the most important buildings of the Armenian church. Since 2000 it is a Unesco World Heritage Site.
hikeminded 
The second and third photo looks quite modern, is that a more recent addition?
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Yes they are. On the area of the Etchmiadzin complex are several buildings from quite different ages. If you are interested, there is an interactive map of them: http://www.tacentral.com/echmiadzin/map_ech_complex.htm
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First photo is the entrance portal, second obviously the observatory.
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