They want another science professor LOR from you? Their website directly states that if you don't have a committee letter, you are required to give in 3 faculty letters of recommendation - however, only
two of those have to be from science professors. I would call them and double check since it's usually easier to get a non-science faculty letter, and you don't want your application to be delayed!
I hope it's only two, since I have two science LORs, 1 non-science LOR and 1 DO and 1 non-academic. I would be screwed if I had to find another science professor.
Source:
"Fulfillment of the Letter of Recommendation requirement which is as follows:
A letter of recommendation from your pre-health or academic advisor/committee (undergraduate, post-baccalaureate or graduate programs).
NOTE: This is the preferred letter of recommendation by the DO Faculty Committee on Admissions.
OR
Three faculty letters of recommendation: Two of those three letters must be from SCIENCE professors (undergraduate, post-baccalaureate or graduate program"
http://admissions.pcom.edu/app-process/doctor-of-osteopathic-medicine-do/