Adding multiple form values

[Jackie hardy]

Hello,

The below works for 2 values: 

{{forms.last.S2Amt1.plus(forms.last.S2Amt2).asCurrency}}

I am trying to sum additional values but it is not working.

I have tried:

{{forms.last.S3Amt1.plus(forms.last.S3Amt2).plus(forms.last.S3Amt3).asCurrency}} and it is only adding the S3Amt1 and S3Amt2. Any suggestions? Thank you.