Hey yo. I'm in D3 right now. You can most definitely carry as an assassin. Yes, you gotta get those roams on. A thing with assassins is you can easily dive enemies under tower. As you go up the ladder, ganking under turret becomes more common and more clean (ex. Challenger junglers aren't afraid to dive gank top lane at level 4). That's just my little add on for ganking.

I always aim to destroy bot turret first. 2 kills better than 1 right? Once you break bot turret, make bot go mid and afterwards, you're gonna split bot. All assassins have an easy escape and can easily kill the ADC if they're alone, forcing the support to also stay while mid is 2v1.