In OpenStudio, the Loads tab is where you set inputs for various internal load definitions (people, lights, etc.). For server racks in an IT room, that would be defined as electric equipment. The electric equipment definition asks for the MAXIMUM electricity use of the equipment, either as a $W$, $W/area$, or $W/person$. You mention that each rack uses 3 $kWh$ of energy, so you need to convert that to a maximum power input (maybe that is energy use each day?).
The other inputs of the electric equipment definition are heat loss fractions -- how the electricity used by the server racks is converted to heat gains within the IT room. For server racks, these should likely be zero for latent and lost fractions -- it's up to you what the radiant/convective heat loss fraction split should be.
Another aspect is the fractional schedule that modifies how much electricity the server racks use over the course of the simulation. This schedule is combined with the electric equipment definition as inputs for the space type in OpenStudio.
The severe error stopping the simulation occurs because there is too much heat being added to the IT room by the server racks -- the IT room's cooling system can't maintain proper air temperatures in the room.
You may be interested in reading a paper discussing prototype energy models for data centers. The authors used the Air-Cooled IT Electric Equipment object of EnergyPlus to model server racks instead of the "standard" electric equipment object.
