The high altitude would change the boiling point but I don't think it would cause the graininess. I don't know what toffee recipe you're using or the directions, but if you add a different form of sugar or an acid it reduces the likelihood of crystallization.

So, for instance, a tablespoon of corn syrup will help prevent the granulated sugar crystals from bonding.

Carol

The altitide makes no difference...it will take longer to come to the right temperature....but eventually will.
3000 is not really high at all.
Linda C