That doesn't make sense!
As others have said, you can't have a constant voltage and constant current with varying resistances. That's such a basic mistake that it casts doubt on your technical ability and clouds the rest of the discussion. This short post does not count as 'due diligence'; you're messing with dangerous stuff here and need to know what you're doing.
What are you doing?
With that said, what are you doing? 1.5-3mA through cranial electrodes doesn't do anything useful or interesting that I'm aware of. There aren't many sensory neurons or muscles on the scalp, and there are far easier and safer places to test reflexes. The skull and cerebrospinal fluid are conductive; you won't get any current into the brain this way.
Here's some recommended reading
I suggest that you thoroughly read and make sure you understand section IV, Electric and Magnetic Stimulation of Neural Tissue, in the free online Bioelectromagnetism book at http://www.bem.fi/book/ by Malmivuo and Plonsey. I also suggest reading the rest of that book. Don't ignore the references cited at the bottom of each chapter - If you want to build a device for this, you should be very thorough in your research and look up some or all of the references.
And here's what you really need to do:
Finally, I'd like to posit that you might not have the skill required for this project. You're dealing with human lives; a mistake on your part could kill someone. You don't want to risk that. The wiser move would be to purchase a system for this purpose designed by people who know what they are doing. I've used ADI Instruments BioLab data aquisition modules before; they include a stimulator module with which you can apply electrical stimuli of various currents and durations. I've applied electrical stimuli with this system to my own body, and I'm still typing this. Had I done the same experiments with low-impedance electrodes, a 9V battery, and a tiny little wiring or circuit design error, I might not be typing right now. Be careful.