It is between all of you to figure out a percentage that works for all of you. There is no set way. Since there are three of you, you could split it into thirds.
You can indicate that you shared the policy and prorate. The pro-rations must add up to 100%. You can also click on the blue link. What percentage of the following policy amounts do you want to claim on your tax return? See "How do I figure out these percentages?" when you get to that screen.
Child/Children on another tax return
If you share a policy with someone who is claimed on another tax return (usually a child), you can take the number of people on your tax return who are covered on that plan and divide it by the total number of people on the plan. This would be your percentage of premiums, SLCSP and advanced payment of the premium tax credit, unless you agreed on a different number beforehand.