Ok, so the selective service registration is not required. Great!
If never activate visa and never travel to the US on immigrant visa, you won't become a LPR.
If you come to the US to activate visa, leave, and don't travel there for 3 years, there's great chance of being stopped at border after 3 years and being referred to immigration court (NTA). At immigration court you have to prove you had ties to the US allowing you to keep LPR status. If you don't have job, don't have property or lease, bank accounts etc in the US, it will be nearly impossible to prove you have sufficient ties to the US. And at that point, judge will likely take status away and you will have to apply for visas to visit the US