Tercentenary Square ist in my opinion part of the Plaza de Armas of Cusco, but do be honest, I do not know where the real difference between both squares is. Probably the Tercentenary Square is only the inner quarter of the Plaza de Armas.... However, it is really beautiful and worth a visit.
Tercentenary Square is a section of the Plaza de Armas in Cusco and also a bustling section of the city. The square has stunning views of the churches and the Statue of Jesus. There are a plethora of restaurants and shops surrounding the square.
This is the Plaza de Armas, Cusco's main square & where most of the tourist activity takes place, nice places for snacks & coffee on balconies overlooking the plaza - gets chilly in the evenings so wrap up if you are going ot be there after 4pm.
Spend your time wandering Cusco and put this square on your list but don't fret if you don't make it to this spot. I suggest having a general itinerary of Cusco sites and wander. You will be constantly entertained and discover new things around every corner.