Postgres 9.6.5 - Can I use a query to calculate a value from two columns and insert the calculated value in a new column in the same table?
I found the below which seems to have done the trick...

Firstly, I added the Value3 column to Table1 using the below SQL:

ALTER TABLE "Table1" ADD COLUMN Value3 NUMERIC;


Then I ran the below update and set statement:

UPDATE "Table1" SET Value3 = Value2 - Value1;


This updates the Value3 column with the result of Value2 - Value1.