You really don't need an excel program to do this.
heat lost by warm water (3000L) + heat gained by cool water (30L/min) = 0
Five minutes into the operation you have 3000L of 115 C water(convert to C and call that Tinitial for the hot water.That part of the equation is
[mass hot water x specific heat H2O x (Tfinal-Tinitial)]
At 30L/min x 5 min = 150L so that part of the equation is
[mass cool water x specific heat x (Tfinal-Tinitial)] = 0
Tfinal is what you solve for.
Tinitial for warm water is 115F (but converted to C). Tinitial for cold water is 1.7C and Tfinal is what you solve for. Put all that together and you have your first point into the 5-min operation. Then just redo the whole thing for the next 5 min cycle. The whole thing looks like this for the first cycle.
[3E6g x sp.h. x (Tf-about 46)] + [150E3g x sp.h. x (Tf-1.7)] = 0
Solve for Tf.