If I run 10/3 wire black and red to a double pull 30 and the other ends to black and red on heater and ground to ground screw and to ground bar what do I do with the white one thanks

IF you must run 10/3 you should just cap off the white on both ends.

Most of us run 10/2 to a water heater because that's all that's needed. There is NO neutral in a straight 240v circuit. Yes, you can use the white and black as your two hots.

Use the black and red as your two hots, not white and black as suggested.

KISS, I was suggesting to use the black and white of a 10/2 cable.
Read the first sentence in my reply.

Leave it disconnected at both ends
Water heaters on need a 2 wire w/ground

Put the black and the white on breaker and at leads at water heater