Make 3 separate tables, or 1 combined table for similar data? When designing a database's structure, if we have multiple tables with similar data, should these tables be combined, with an added ENUM field to denote the table type; or should 3 separate tables be created?

E.g.


VAT (sales tax) rates
National Minimum Wage rates
Currency exchange rates


The structure of the tables would be:

|============= VAT rates =============|
| id | rate |    from    |     to     |
|----|------|------------|------------|
| 1  | 17.5 | 1991-04-01 | 2008-11-30 |
| 2  | 15.0 | 2008-12-01 | 2009-12-31 |
| 3  | 17.5 | 2010-01-01 | 2011-01-03 |
| 4  | 20.0 | 2011-01-04 |    NULL    |
|=====================================|


All 3 tables are frequently referenced by queries.
The full keys to your data are more complex that you indicate.  Sales tax rates can vary by jurisdiction and goods can be categorised as luxury or exempt. Wage rates can vary by industry and public holiday / normal work day.  Exchange rates have to be keyed on the source and destination currencies.
While your values may all be fractional numbers they have different units, which makes them different things.  Tax is &quot;percentage&quot;, wages are &quot;dollars per hour&quot;, ForEx rates are dimensionless ratios.
Your data scope my be constrained to eliminate these considerations, of course.  Only you can say.
Joe Celko covers this topic in depth here.