Yes, you can buy 3 equal speakers for the LCR channels if you like.
Just adjust the numbers in the cart, like shown below.
This can be done for all the speakers in the 1723 Series. For the 1961 Series, this is possible for the 1961 Tower only. Here is why not.
Please note the 1723 Center (S) is equal to 1723 Monitor (S) except for the orientation of the back panel with the binding posts. And the logo on the front. The logo on the metal grille is magnetic and can be moved around as you like.
For the 1961 Monitor, the 1961 Center is equal to this.