How to select a numbered group of elements

I have an xml file like (there can be any number of <tx> elements)

<payment>
 <tx><paymentId>123456</paymentId><payType>EURO</payType><amt>10.00</amt></tx>
 <tx><paymentId>123457</paymentId><payType>EURO</payType><amt>20.00</amt></tx>
 <tx><paymentId>123458</paymentId><payType>EURO</payType><amt>30.00</amt></tx>
 <tx><paymentId>123459</paymentId><payType>EURO</payType><amt>40.00</amt></tx>
 <tx><paymentId>123460</paymentId><payType>EURO</payType><amt>50.00</amt></tx>
</payment>

which I need to group in sets of 3 so that I can output a checksum line every
three lines of the <amt> tags. I also need to do this for the last line as well

eg
Header Text
123456    EURO 10.00
123457    EURO 20.00
123458    EURO 30.00
Checksum  60.00
123459    EURO 40.00
123460 EURO    50.00
Checksum  90.00
Footer text

Is this possible with XSLT

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