The effect will be the same...the deduction ends up on Line 29 of the 1040 (as you already know). I would split the premiums three ways and put the expense on the three different businesses. This way, the total shows up on Line 29 of the 1040, and all the businesses are showing an expense for the health insurance payments.
Unless of course you can trace the premiums payments to one business in particular, which is completely acceptable also.
