Longoria, Zimmerman, Gay-Rod, Wright, Beltre, Young, and Youk are probably the top 3B in the league overall(I could be missing someone, this was off the top of my head). That's 7/30 of the starting 3B i the league. I think it's safe to say they will be above average at 3B this season.
If I had to make a list taking into account offense, defense and age it would go like this,
Longoria
Zimmerman
Wright
Youk/Arod
Beltre/Young
Scuataro needs to start in the beginning to keep his trade value reasonable. Allow Lowrie to ease in and not throw everything in his lap at once. If he stays healthy and is hitting, they can always move Scutaro(decent SS should get something of value toward the TDL) for other needs later on. Next season he can get the majority of the time at SS t start off with Iglesias coming in a few times a week as they ease him in to the ML level. Lowrie in those games ca give the other IF's an occasional breather.