If possible a good time to vist would be sunrise on the first day of the four seasons as the rays enters the southern temple of Mnajdra and lights up its interior - that must be a great sight. On the equinox days, sunlight not only enters the temple but its beam also lights up the temple’s main axis.
The temples are opened specially for these events apparently - check the heritage Malta events site for more details.
The third, with its largely intact façade and bench constructed in the early Tarxien phase is perhaps the most impressive part of Mnajdra and perhaps the finest surviving on the Maltese Islands.
The middle temple was the last one to be built, dating to after 3100 BC. It was inserted between the other two, and set at a higher level on a sort of terrace. Unfortunately this is mostly modern reconstruction seen here.