I have a 240 volt heater 2 wire (plus ground).
I want to temporarily use the heater out in my shed . I have run 10/2 cab tire cord from the house to the shed.
Now at the house end I would like to plug the 10/2 cord into my 30 amp dryer receptical. I have a spare 4prong plug from an old dryer. Can I join the end of the 3 wire plug to the 2 wire heater cord using only black to black , white to white ground to ground and then cap off the third red wire from the dryer plug ?

What is a cab tire cord? What is temporary?

With 4 prong plug the voltage is 120 black to white, 120 red to white and 240 black to red.

You sound dangerous -to yourself -if you are going to start plugging things in. 4 prong is a 120/240 as is a 3 prong. For 240 you need 2 hots and a ground with a double breaker.