You want to drop 30V DC down to 27V DC with a load of 6 amps?
A few diodes in series are a simple option.
Go to Digikey's parametric selection system:
http://www.digikey.com/product-search/en/discrete-semiconductor-products/diodes-rectifiers-single
Check the "In stock" box. In the "Current - Average Rectified" column, select everything from 8A to, say, 20A (hold down Shift and use the arrow keys to select multiple lines). In the "Mounting Type" column, select the mounting types you want - you'll probably want to avoid surface-mounted devices. Click "Apply Filters".
Below the Apply Filters button, in the Quantity field, enter 1 and press Enter. This will move the devices with high minimum order quantities to the bottom of the table. Then sort the table on whatever parameter you're interested in - e.g. price, package, etc.
Look in the "Voltage - Forward (Vf) (Max) @ If" column to get an idea of the amount of voltage the diode will drop. The value in this column is a maximum value, not a typical value, so download the data sheet for any diode you're considering and look at the typical forward voltage vs. forward current graph. Voltages in series add together, so you'll need to choose a number of diodes that add up to about 3V.
Power dissipation in each diode is equal to the current through the diode (6A) multiplied by the voltage dropped across it. A few watts will make the diode pretty hot and they will probably need heatsinking. TO-220 packages are easier to heatsink than the cylindrical ones.
Other specifications aren't really important. High speed is irrelevant for this application but isn't a problem. Standard diodes have a voltage drop of about 1V; Schottky diodes have a significantly lower drop, but are sensitive to reverse voltages; I would connect a resistor (e.g. 10 ohms) across each one, if you use Schottky diodes, as protection.
If your load current is very constant, you could just use resistors instead of diodes. To drop 3V at 6A you need 0.5 ohms; this could be made from a number of higher-value resistors connected in parallel. Total power dissipation will be 18W.
