Three hole vanity sink

The sink in question is from Kohler and is a cast iron vanity sink with three holes. They say it has drilling for faucets with 4" centers. Does this mean I can't use a single faucet and put in a soap and loation dispenser alongside the faucet?

Hard to say. Its likely that the 3 holes are all spaced 4" apart, in which case you would have no problem installing tthe items you want as long as the faucet can be mounted in one hole and doesn't require the large mounting plate.
If they have 3 holes lined up with the 2 outside ones at 4" centers and one in the middle you will be very hard pressed to fit the 3 items in.

Three holes with 4" on center between the outside holes is for a regular center-set faucet.

Not recommended is to very very carefully chip away the porcelain down to the cast iron, and then drill or holesaw your new hole.

Can you drill through the counter top?

sink

It would be very unlikely that the underneath bottles for the two items would fit because of the faucet body underneath, even if there were room on the top for the escutcheons for all three not to overlap. To do what you want to you need a sink with an 8" drilling.