Mapping Logic: Value of source field to be output on another sequence of target field

Hi Guys,

I would like to seek for your expertise.

I have the following source XML format:

<SourceMessage>
     <row>
          <tax></tax>
          <amount></amount>
     </row>
     <row>
          <tax></tax>
          <amount></amount>
     </row>
     <row>
          <tax></tax>
          <amount></amount>
     </row>
</SourceMessage>

For my target message, I have the following XML format:

<TargetMessage>
     <TargetSegment>
          <amount></amount>
     </TargetSegment>
     <TargetSegment>
          <amount></amount>
     </TargetSegment>
     <TargetSegment>
          <amount></amount>
     </TargetSegment>
</TargetMessage>

If my tax field has a value from the source XML, the amount will be produced on the <amount> field under the 3rd occurrence of <TargetSegment>.
And the other <amount> fields which do not have value or null for <tax> will be produced in 1st and 2nd occurrences of <TargetSegment>.

Example:

If my second occurrence of <row> has

<tax>123</tax>

<amount>1234</amount>,

the amount will be produced in the <amount> field under the 3rd occurrence of <TargetSegment>.

How can I create this logic in my mapping?

Thanks and Regards,

Neil