Hello, 

As per your question, i would say that there is no 3rd phase counseling for ap ap eamcet 2018. As you have got your college already as per your rank, you have to take admission there only or again wait for next year exam, appear it and score more marks and get your preferred college. This year you can only get admission in your alloted college as no more round of counseling is left. 

For more details please check, 

https://engineering.careers360.com/articles/ap-eamcet

Good luck