what voltage will 7805 deliver as output if input is lower than 5 volts, lets say 3 volts
You can see typical behavior shown in the datasheet: 



Around 3V in it will start to turn on so the output voltage will be fairly unpredictable with that input voltage (probably will vary from unit-to-unit and with temperature). 

It sounds like you want some kind of supervisor functionality. There are many such chips and there are a few regulators that provide an output "power good" signal when the output voltage is fairly close to regulation.