The simplest method would be to do a quick calculation. Determine the rise time using a single RC "element".
For one RC "element", the impedance is Zc = R - (1/jwC), where w (omega) = 2*pi*freq
To calculate the parallel equavalent (Ztotal) of two RC "elements", use 1/Ztotal = 1/Zc + 1/Zc
for three, it's be 1/Ztotal = 1/Zc + 1/Zc + 1/Zc, etc, etc.
With a little algebra, you can calculate the effective parallel impdeance, and from that, you should be able to determine the equivalent circuit and calculate the new rise-time, and comapre it to the original rise-time.