What a great, tough question; dependent on so many factors...
I can see Seattle executing a Patriots approach to their running back corps, picking their backs based on favorable matchups.
On pure rushing yards, it has to come down to Lacy or Rawls, seeing as Procise is a far greater threat catching the football.
Obviously durability is a factor, with all three backs being injury prone.
Additionally, I believe the bigger Lacy needs a greater head-start than Rawls, meaning the line can not be a mess for him to succeed.
Rawls never really was healthy last season, and he only got his full cutting strength back towards the end of the season. This year, IF he can stay healthy, he should be much better.
It really depends on who gels with the line the best in my opinion, we saw the effect of that with Rawls in that 2015 stretch of brilliance.
I'm going to go with Lacy. If this happens, the line will be good at run-blocking and we'll have a perfect change of pace compliment in Rawls waiting behind him.