Letter from a PI v. Someone I have worked with closely

[Halek]

So I'm applying to Ecology (and a few earth science) programs for 2018. I'm currently working as a field tech, and will be getting a letter from my boss, who is a faculty member at a university. For the second letter I will most likely ask a professor I volunteered with right out of undergrad, between graduating and finding my current job. I'm in somewhat of a pickle for the last one. I am trying to decide if I should reach back to connections I made during undergrad (I graduated in 2016) or if I should as for a letter of recommendation from a supervisor at the field station I work at, who I work with pretty constantly and has first hand experience on how I approach solving problems in field work. But he doesn't have a faculty position, so the letter might not look as "good". Adding to the confusion, I will be starting a new technician job in a few weeks with a new boss, and while he might be able to provide a good letter in the future, I'm not sure if that will be the case by the time applications are due.
TLDR: I have two letters sorted out, and am trying to figure out the last one. 

[TakeruK]

I think you have two really good letter writers already so it's not as important if the last letter writer doesn't tick off all of the boxes of professor/academia/knows-your-research. 

If your field station supervisor has a PhD then I would say this is certainly the right person to have as your 3rd letter writer. If not, your field supervisor may still be better than going back to a prof from undergrad that doesn't know you as well.

[AdmissionsTrack]

If this supervisor has an advanced degree you should use him. As long as he can state that you have the drive needed to complete advanced training based on his own experiences, the LOR will be respected. - Admissions Track