Edit 2: Just tested and all 3 bolts can hit the same target, at least if the hit box is big enough.
I've done some additional testing and found that Weapon Throw: Mighty Throw kills Ghom faster than Frenzy: Smite EVERY SINGLE TIME.
Mighty Throw kill time: 35 +-1 s
Smite kill time: 38 +-1 s
My SOH Charged Bolt Damage = 230
Theoretical Frenzy damage (5 stack) with SOH: [155 + (230 x 3 x 0.75)] x 1.75 = 1177
Theoretical Mighty Throw damage: 270 + (230 x 3) = 960
Since Mighty Throw outperformed Frenzy even though it had lower theoretical damage output, it would appear that there is a cap on how often SOH can proc (=Internal Cooldown). Now lets take a closer look:
Observed Frenzy: Smite damage = 35/38 x 960 = 884
884 / 1.75 (frenzy AS bonus) = 505
505 - 155 (smite damage) = 350 (SOH proc damage)
350 / (230 x 3) = 0.51 = 51% (SOH proc rate observed during my test)
This means that SOH only procced ~50% per hit instead of 75%.
50/75 = only 2/3 of the hits that "should have procced" actually procced.
My APS: 1.68
Frenzy APS at 5 stack = 2.94
2/3 = x/2.94
where x = Max number of SOH proc per second, capped by game
x = approx 2
Conclusion: SOH can only proc max 2 times per sec.