can i join electrical engineering after completing computer three year diploma with first class

yes surely u can join but u will face lots of problem as u have the base of computer in diploma. I will suggest u that if u have done ur diploma after 12th then u join electrical engineering else cont. in ur own field , either go for IT or CS it will be more helpful to you

No not possible.

Yes you can join B.tech Electrical or Electrical & Electronics Engg as a Lateral Entry student. Best of Luck