Haven't been to Lviv. Been to Kiev and Krakow, both are worth visiting.

Kiev is less touristy, bigger and more off the beaten path than Krakow. Kiev has plenty to do for three days (I am assuming you mean 3 days, 4 nights stay). The op states that the three cities you're looking at are inexpensive - wrong; accommodation in Kiev is expensive.

Krakow's city centre is small and can easily be seen in a day or two, if you stay for three days you may want to consider a day trip to Auschwitch or the Wieliczka Salt Mine.