Honestly, I'd do the 3 days a week. Both of my children dealt with speech delays (9y/o graduated out of speech therapy & 4y/o still in it). I know it's less convenient (we also only have one vehicle) but it would be to her benefit.
Is riding the bus an option for her so that you do not have to drive to and from preschool? Or maybe talk to other parents to see if anyone in your area would be interested in car pooling. One could drive to school the other after school.
Good luck with whatever you decide.