"so my question is that, is it possible to use an 555 timer to generate these signals along with inverter to generate approxmitally 469 KHz"

Yes you can. The clock frequency can lie anywhere between 10 and 1280 kHz. But somwhere around 500 kHz is just fine.

Since you want to read three channels, you have to get control over the address,ALE,EOC, and SC pins.

Appears you are using 8255 also. I would simply say that the set up with 555+8255 may result in a kind off proving-the-concept setup. The end results of the setup may be far from what one would expect from a 8 bit ADC output. ( Kai has already enumerated the reasons for this )

OK when you finally do manage to put everything in place do this simple check :

*Connect the sensor input lead to ground AT the sensor end after removing the sensor. (It is important to have the actual connecting cable in place )

*Setup a looped ADC reading for that channel and update the output (raw data )once per second on the display device .

*If all is well you should not see anything other than a "00" or an occassional "01". If you see any other value that keeps rolling, jumping etc then the setup is not optimal. And mostly the problem is with the ground.

Raghu