I need some help on how to add taxes to newly purchased vehicles, according to which for the first three years after the purchase data the taxes are higher. After those three years, the taxes are then lower.

Taxes that are dependent on the age require defining a SHAPE index, and then applying that shape either directly to the tax (fixed tax on capacity) or to a parameter defining a dummy flow for the taxes (tax on a flow).

For fixed taxes this is thus quite easy: just define the NCAP_FTAX (the tax) and NCAP_FTAXX (the shape index for that tax). For taxes on a flow it is slightly more complicated, and the process must then also be vintaged: First define the dummy commodity and the dummy flow e.g. by FLO_EMIS(source,dummy) = 1, then the FLO_FUNC(dummy,dummy) = shape index (for shaping the dummy flow by age), and then define the tax on the dummy flow by FLO_TAX, with a value that gives you the correct tax when multiplied by the shape.

There is an example on the VEDA Forum for defining an age-dependent flow: Defining dynamic emission factors.
While the example given there just defines an age-dependent CH4 emission flow proportional to the activity, it would be trivial to add a constant flow tax on that emission flow, and thereby you would get an age-dependent tax on the activity.

Thank you for your response. Still, I am having some difficulties to fully understand how I should implement this extra tax on the first three years after purchase of a new vehicle, starting after 2022 for all diesel and gasoline vehicles.

This is what I have so far, but I am still missing to set that this tax is just applied to the first three years from the purchase year. I am missing to define the shape index, but how can I do it? Am I missing something more?

Ok, you are defining a fixed tax, which is the easy case, as I mentioned already.

You just need to define a SHAPE index and a linking NCAP_FTAXX, as follows: First choose any vacant number (2-999) for the SHAPE index, e.g. 33 (if that is still unused in your model). Remember that index 1 is always reserved. Then define the SHAPE profile:

As you can see, the value is 1 for ages 1 to 3 and then zero. When these multipliers are applied, you will thus get a tax only for the first three operating years. (Under VEDA, the index number goes to Other_indexes, and the age goes to Year.) Then you still need to define the linking NCAP_FTAXX, which defines the SHAPE index to be used for the NCAP_FTAX: