One base cake mix and then make several flavors

Adjusting a basic cake (either from a boxed mix or from scratch) is easy if the cake has no or very little flavour of its own.

Using a boxed mix might be more difficult because they often have a generous amount of flavouring, typically vanilla, even if it does not explicitly say so on the box front. Check the ingredients list or use a mix you know - a little vanilla is ok.

You can either prepare the basic recipe, then divide the batter (I'll call this method 1 for now) or divide the ingredients and make separate batches (method 2). The latter might be difficult if you need to split eggs, but it's possible. See this question.

Now for the adjustments you asked for, amounts suggested should be enough for 1/3 to 1/2 of normal recipe:

• Vanilla bean
  Assuming you want real vanilla, split 1/2 vanilla bean and scrape out the seeds. For method 1, add seeds to finished batter (to avoid lumps, mix with a few pinches of sugar until "sprinklable") or stir into liquid according to recipe for method 2.
• Chocolate
  Add 1 to 1 1/2 tbsp of cocoa powder (the real stuff, not drink mix!) either to the dry ingredients or flour (method 2) or sive over the batter and stir (method 1). You might have to add a tsp of extra liquid - or not. Hard to say w/o knowing the original recipe. Do not use normal chocolate, because this may ruin your batter, especially if you started from a sponge-type, pound cake is a bit more forgiving. You can make chocolate cake with molten chocolate, but this needs more adjustments / a different recipe.
• Strawberry
  A strawberry taste can only be done with artificial flavouring. I have never used this, so I won't give a recommendation here. Read the label.
• Bonus: Rum, Whiskey, Brandy,...
  Replace ca. 1 tbsp of liquid with liquour of your choice (method 2) or add 1/2 tbsp at the end (method 1). Poking the baked warm cupcakes a few times with a skewer and drizzling with 1/2 to 1 tsp of alcohol (or alcohol with sugar syrup) might be considered method 3.
• Bonus 2: Spices
  Don't forget the powers of your spice cupboard: Cinnamon, nutmeg, cardamom, even ground cloves, give a lot of flavour w/o the danger of throwing tipping the dry-to-liquid balance of your recipe. A very small pinch (cloves, be careful with these!) to a shake or two (cinnamon) is all you need.

Note that you can mix-and match the methods above, e.g. add one tbsp cocoa and 1 tsp of brandy or dissolve the vanilla bean scrapings in a bit of of whiskey. Chocolate goes nicely with a pinch of cinnamon, you could even go all "Christmasy" (ha, and to post this on Easter...) with gingerbread spice or brandy, cinnamon & nutmeg for "eggnogg".

To add to Stephie's great post:

Strawberry: Strawberry Nesquick Powder can be added to white/vanilla boxed cake mix and it will turn out okay. Just one of those "if you have it on hand, it can work" things. I still prefer just buying a box of strawberry cake though, the flavor comes through stronger.

Some recipes that use it:

Nesquik Neapolitan Pound Cake

Strawberry Milkshake Cupcakes

My response is regarding the Chocolate addition of the three cake flavors. Just keep in mind when adding cocoa powder to a cake recipe (that is assuming it is a scratch recipe), you have to treat the cocoa powder like flour. Adding it to a recipe without subtracting the amount of flour equal to your cocoa powder will result in a dryer cake.

Ex. If you have a recipe that calls for 4 cups flour, you want to add 1/2 cup cocoa powder, you must remove 1/2 cup flour from the recipe. This will ensure your cake comes out as predicted...delicious!

Note When using a box mix of cake, I have not personally tried adding cocoa powder. Since all the leavening agents are already mixed into the bag, removing any of the mix of flour for equal amounts of cocoa can alter your cake possibly for the worst. My response is simply for making a from scratch cake recipe.