We demonstrate experimentally that several classes of phase functions are capable of enhancing the amount of three-photon fluorescence from molecular iodine by at least a factor 3 with respect to an unshaped pulse. The investigated functions include third-order Taylor expansions, π-steps and highly structured forms identified by an evolution strategy. The mechanism of the observed control is investigated using wavepacket simulations and found to be due to a time-delay in the second resonance of the excitation ladder.