3 inch slugs generally increase velocity, which can be a plus or minus.
For well constructed sabots, increased velocity means a flatter trajectory and longer range.
For cheap lead foster slugs, the increased velocity means an increase in the fragmentation range, generally a bad deal. Foster lead slugs tend to blow up at high velocity, and still lose stability and accuracy at long range like their lower velocity counterparts. They are so un-aerodynamic that the increased velocity doesn't really increase their effective range; it just increases their short range fragmentation range. Even "standard" velocity foster slugs already blow up at short range. The new Federal deep penetration slugs, you will find, actually have less than standard velocity.
In a nutshell: 3 inch sabots=good, 3 inch rifled slug= not so good.