Looks like your tap will exceed 10 feet in length (from the picture). If so, you will need OC protection at the disconnect.
Also, the split bolts may not be rated to connect #3 and #12. As I recall they go from #8 to #2.
"Madison Equipment UL listed for copper conductors only Silicone bronze construction Size/ min/max range of equal main / tap Color: DESC:Solid Size: SIZE:8/4 Mfg Part Nr: SB-4"
You could run #8 from the split bolt to the fused disconnect (or CB enclosure), then #12 from that point on.
