How do I solve a maximization equation with 3 unknowns?

for example the utility function: U = ln(C1) + ln(C2) + ln(C3)

s.t. C3= (1+r2)(1+r1)[Ybar - (C2/(1+r1)) -C1]

I'm trying to find optimal level of C1, C2, and C3?

Thank you in advance!