Great question! If you're looking for your customers to have the ability to build different options for their package you'd be looking at an app that allows for product options. It isn't something that comes natively in the Shopify Admin, but we have some great options for you in our app store that you might want to take a look at.
This will allow you to customize your product page to allow your customers to decide what products will be included in the 3-pack or 5-pack.
The other option would be to reach out to the Shopify Experts to see if they could build something like this for you. This would be a more expensive price in the beginning, but in the end, it may be cheaper than a month by month rate to use one of the apps. I can't guarantee that the Shopify Experts would be able to do this for you, but they're very talented when it comes to different features and customizations that it really is worth looking into.
Can you take a look at those options to see if they'd work for what you're wanting? Please let me know if you have any further questions.