The 3-PL b values are roughly equivalent to the Rasch difficulties. The b difficulties are probably in probits, which could be converted to logits by multiplying them by 1.702. However, empirically, the conversion factor differs.

 

Do you have any of the 3-PL data? If so, use the 3-PL parameters to estimate the 3-PL thetas for those data. Then analyze the same data with Rasch and obtain the Rasch person measures (Rasch thetas). Cross-plot the two sets of thetas. The slope of the best-fit line gives the conversion between the empirical 3-PL scaling and the Rasch logit scaling.