Almost 3 And Still In Diapers

   

I need help, my 2yr old twins are going to be celebrating their 3rd birthday in about three and a half months yet they still don not alert me when they have to poo or pee... 

 They speak out for everything else though.....  I have tried,  "The Cold Turkey Method", no diapers walking around in under wear like big kids and I end up cleaning up dung off the floors and carpets...

 Not to mention urine....

    I have also tried to stick them on the  potty for extended/repeated periods, which   ends up with them pooing in the new diaper I put on them after I take them off the potty....   

How can I accomplish this???