What the three readings are
Each reading is the slope of one part's centerline in side view, off a magnetic angle finder: the output yoke, the driveshaft tube, and the pinion (yoke face, or across the diff cover bolts). Positive if the part runs downhill toward the rear, negative if uphill. Measure on level ground at ride height, then again loaded; the angles move with weight.
What Spicer says
For a shaft with a single U-joint at each end, Spicer's rules (J3311-1-DSSP): the two operating angles should be equal within 1 degree so they cancel, each joint needs some angle so the needles roll (the guide says 1 degree, Spicer's calculator accepts 0.5), and neither may exceed the limit for the shaft's speed. That limit drops from 11.5 degrees at 1,500 rpm to 3.2 at 5,000, so the tool asks for speed, axle ratio and tire diameter. For a double-cardan (CV) shaft, Tom Wood's says to point the pinion at the transfer case so that joint runs near zero.
Reading the result
"Pinion setting to match" is the pinion reading that makes both joints equal (single U-joints) or puts the pinion in line with the shaft (CV). With output 3.0 and shaft 8.0, the front joint runs 5.0 degrees, so the pinion wants 3.0 too; shims or adjustable control arms get it there. A common owner practice on leaf springs, not a Spicer rule, is to set it a degree or two lower because the axle wraps up under power. If a joint stays over the limit after a lift, that is a driveline shop job.