Depends on your riding styles...Commuting/racing/ bad road conditions might be a concern...First bike..Is a Sunday road bike for special ocassions. Maybe when you crave faster rides..
Second..For me commuting or touring..Those can often be grouped together...A third bike might be...Mabye riding in the rain or into high crime areas, where you don't trust your Kryptonite locks. I think four bikes is not too many. Serves most of my needs...Except for fourth beater bike is temporarily out of commission.