epr2npr

Currently a first-year MEng student in BME applying for PhD programs. My first two letters of rec are from - 1. PI from lab I worked in over junior summer, published a couple reviews with and 2. Advisor for senior design project I have a few options for the third letter, none of which are particularly appealing but probably would be workable. 1. Professor I was a TA for in undergrad, also took classes with, etc 2. Head of undergrad BME department 3. Head of MEng program at current school (I feel strange asking for this one since I've just started the program in the fall) Again, not particularly thrilled about any of these options but I do need a third letter of rec for most applications. For those which don't require the third letter, should I omit it?