I'm sure you must know this answer, but...
Define 3 build modes in Lazarus project options.
For each mode in project options,other,custom options, define the relevant defines such as -dPRERELEASE -dWHATEVER -dSOMEMORE
... IIRC somebody also recently posted a method where you can use .inc files but you'd have to include these with all your source files.