I had this problem when I first started out. I have kits in three sizes: Sm, Med and Lrg. This is what I did to make sure they chose something:
Under product description, I set the base price to 0$. I also set the base weight to 0.
I created the Option Name (Size), and set it to a Drop Down Menu. Attached to that, I added the three values (Small, Medium, Large).
In the Attributes Controller section, I added each of these three to the product. This is where I set the price and weight of each size kit, plus the sort order and clicked Yes on "Include in Base Price" box.
On the Medium size attribute, I also checked "Default Attribute to be Marked Selected".
Now, when someone goes to order a kit, if they just hit Add to Cart, they automatically get the Medium Kit. When they see it in their cart, and if it's not the size they wanted, they backtrack to find where to make the right choice. I haven't had any complaints with this method yet.
Now, if I had the kind of product where they wanted to order a quantity of three of the same thing, in the three different sizes, they would have to go through the "Add to Cart" process for each different size. If they wanted two medium and one large, they would have to go through the Add to Cart process twice. Radio buttons allow only one choice at a time.
Now, using Check Boxes might work for you. They allow the buyer to check any and all of them. If, in the product description, you make your product 0, check yes to Price by Attributes. When you create the attributes, put in the actual price you want for that size or color or whatever. Then on ONE of them, make it the Default option that will be checked.
People will then see all the options layed out in front of them, see one of them is checked, and they can then check another one and uncheck the default one. If they don't uncheck the default one, it will add that amount in the total (even tho they choose a quantity of 1), and they will say "Whoa, why is that so high?" and go back to make sure they made the right choice. In this way, they can choose one of each option, or 3 of one option if they remember to up the quantity to add to cart. Not perfect, but I've shopped at stores with much less user friendly carts.
Hope this helps!
Patti