I have searched my resources on turfgrass looking for what might cause a condition like this.  None of the diseases seem to fit.  I don't think it's an insect.  I wonder about a chemical spill or an animal urinating on those spots.
When did you first notice it?  Has it changed since you first saw it?  Is it spreading?  Were all 3 spots there at the same time?  When you pulled up the clump did the grass blades come up easily or not?  What fertilizers and/or weed killers have you applied recently (and when)?  How much do you irrigate and when?
I'm going to suggest you contact MSU Plant & Pest Diagnostics (
Plant & Pest Diagnostics-MSU).  Go to the 'contact us' tab and send in photos to their email address asking for plant identification.  Use the tips in the following article to take better photos to submit (GRASS PHOTOGRAPHY TIPS FOR ID-MSU) and provide answers to the above questions in your email.  Hopefully they can identify what's caused your spots.
I'd appreciate it if you would reply to this email letting me know what they tell you.  Thank you.