I see an S3X5.7 (the smallest S3 section) that has a weak-axis section modulus of 0.383 in^3. Assuming 36 ksi steel and that it only supports your 100# at 12" eccentricity and at mid-height (mid-height nullifies any P-delta effect), I get a demand-capacity ratio of 100#/[(0.383 in^3)(36000 #/in^2)/(12 in)] = 0.087. I can't imagine those 100 pounds could fail your column assuming that it is indeed an S3X5.7.
As for your connection detail, don't tap the holes, just bolt through to a nut on the opposite side of the web. That web is probably paper-thin, so tapped threads will limit the bolt to far below its rupture capacity, and drilling 3 holes at 5/16" through the web shouldn't hurt the column any more than 3 holes at 1/4" (negligible either way).
Also on the connection, I would strive for sufficiently thick blocking between the flanges so that the tightened bolts leave an unfilled gap between the flange tips and your outer board. Otherwise you could end up transferring the load by friction between that outer board and the flange tips, where any slippage would activate the bolts in bending (bad) instead of shear (good). And the strength of the connection depends on the distance between the top bolt and the bottom bolt, so try to maximize that distance (8" between 1/4" bolts sounds more than adequate to my intuition). Oh, and skip the glue.