I am arriving in Dubai terminal 1 on Ethiopian airlines and wish to transfer to Emirates flight at terminal 3, do I need to go through immigration/security? do I need to collect my bags?
You do not need to go through immigration, you can walk between T1 and T3. As for your luggage, it is a different story.
If your luggage is tagged to Dubai, then you will need to either collect it yourself (which requires a visa), or pay for the Marhaba Services and have them collect and re-check it for you on Emirates.