Optimal Motor Voltage

I am looking to understand optimal voltage for my motors.  From what I understand voltage is dictated by motors and then multiplied by driver specifications. I have 57HS76-2804-05 motors with ST-4045 drivers.  The spec sheet on the motor is 3V 2.8A.  I can't find any optimal multiplication on the drivers specs for motor voltage.  I run one power supply at 25volts (specs say output can be up to 20amps).  My drivers are set to 3amps with switches. When all three axis are moving I get errors or 2 axis many small movements and wondering if my voltage is wrong.  I understand the 1.89N.M of the motors is not large and the z weight might be pushing the motors capability but want to understand voltage and amp draw before I make that conclusion.  Each axis moving individually works fine and precise.  Multiple axis over time get missed steps and the z can drop in height during movements so before I get better motors I want to understand my errors.   Yes I need to do the math on weight of my gantry and z axis and calculate needed motor strength which I am currently trying to do but understanding optimal voltage is something I need to understand also.  
Any advice or input would be great. 

Thank you again.  Everything you said does align with what I've been able to read on the web including that there is a lot of bad info.  Also the more I learn it is very apparent that you get what you pay for when buying parts.  

As for the errors I get.  Since I've built the machine from scratch I have encountered a lot of errors but I'm down to just one that stops me from using the machine for what I want to do right now.  I want to do some wooodworking to start with.  I ran my first 3.5 hour program (a 3d roughing pass) so single z movements and x y clearing.  It did that correctly for the first time (I was happy).  I tried a 3d finishing pass so many movements all axis.  The z axis either took extra steps down or lost up steps.
This error makes me think it is either to heavy for the motor although test axis up and down for 10min works no issues or I am losing power to the motor.  I wanted to try to understand the total lack of specific power supplies out there for motors better.  I am going to first try to increase voltage to say 36 and run the 3d finishing pass.   It messes up not long into it if it's going to mess up. 
My trial and error method is going to be: increase voltage, increase driver amp setting, run two power supplies

I am happy I have had all the problems I've had so I could learn what I have about self built machines.  This forum has helped me understand so much above Linuxcnc so many thanks again.   

old cnc pro say me use max voltage what you driver can use, him machine used 90 volt. max at him motor drivers and working well last 10 year. volt no matter but current have important use current setting less than motor max current have. this is important. or burn motors. my machine i used 36 volt because have cheap DM556 drivers and i not want use normal 48 volt power supply because fear broken drivers. very expesive 30$/axis.

I agree with your cnc pro. I would use 48v minimum  on say DM542 and 60-72 volts if the drive supported it
I have also run drives at 90 volts with high quality drives.
I don't think you will damage your drives or motors at high voltage. Even my cheap Chinese motors were OK at 90 volts. 
The drives (and probably your drives) usually have thermal overload protection but will probbaly massively miss steps befor they stop.
Motors are only working hard when accellerating