As KSHaggag says the Al Reem lounge is a very poor substitute to the Etihad premium lounges for Gold and Silvers travelling in economy.
You do need to check as not all Etihad flights leave from T3, they still have a number departing from T1, mainly to the Indian subcontinent and occasionally the UK flights.
With regard to the walk, it is only 5-6 minute walk but you have to watch at peak times. You need to pass through security again and the queue can build up back towards T1. What you need to do is walk to the left of the queue towards security and you will come to the fast track lane for business class and Golds. I know Silvers do the same and nobody seems to bother.