The C/L/R 3000 is an awesome center !
normally you want the fronts and centers to match to maintain timbre (seamless front sound) and a third 7001SC would perfectly match the fronts but as others said you might get muddled dialog.
Besides that the C/L/R 3000 is probably the best center available for under $30,000 far better than its MSRP price of $1099 would suggest.
I think you are making a very wise decision sticking with the 3000 C/L/R
Peace and Good Sound,
Forest Man