If I get 3 degrees of success when shooting a combat shotgun on semi-auto at point blank range do I get both the two rounds hitting and then two hits worth each round because of the scatter effect for a total of 4 hits? Or does that not stack and I should only go semi when I'm not at pub range?

On Semi-Auto it is easy: Every time you hit with another bullet you additionaly trigger Scatter. Should you get a really high BS and roll low those extra-hits can trigger Scatter as well.

You might want to avoid the Vanaheim shotgun from the Inquisitors Handbook, that beast goes full-auto. And that makes it tedious to calculate all the hits and scatters.

The Errata actually walks you through an example of this.

Personally we changed Scatter to it's Black Crusade version, where it gives +10 to hit on Short range, and also +3 DMG on Point Blank Range. Makes it useful for low-skill fighters in close quarters and still something of a heavy hitter without going completely overboard.

Verbatim from the errata:

The Actions section starting on page 190 should include a
special note concerning combining semi-auto and full-auto fire
with the Scatter quality, which reads “When firing a semi- or fullauto burst at point blank range with a weapon that has the Scatter
quality, the extra hits for rate of fire and scatter are worked out
separately and both applied. For example, Horatius Kane fires
his combat shotgun at Heretic X. Kane is at point-blank range
and fires a semi-automatic burst. Kane rolls 01 with his modified
Ballistic Skill of 70 (30 BS, +30 for point-blank range, +10 for
firing semi-auto) and hits by an amazing six degrees of success. He
gets one hit at 70, one hit for semi-automatic at 50, and a third hit
for semi-auto at 30 (he does not get a fourth hit at 10, because the
combat shotgun’s rate of fire is 3). He would get additional hits for
scatter at 50, 30 and 10, for a total of 6 hits on Heretic X, most
likely shredding the cultist to bits in the Emperor’s name.”