You cannot use floor() with multiple fields, nor would it make sense to. floor() rounds down a single numerical value.
What you seem to need is the max() function. However, the max() function only accepts one XPath argument, so you have essentially two options:
Put the numerical field in a repeating group and reference it by its path:
So if I have a data source like this:
myFields
group1
group2 (repeating)
valueNode
I would use the formula xdMath:Max(/my:myFields/my:group1/my:group2/my:valueNode)
Arrange the data source so the value fields all have the same name, and use a more general XPath expression:
So if the data source is like this:
myFields
group1
valueNode
group2
valueNode
group3
valueNode
I could use the formula xdMath:Max(/my:myFields//my:valueNode)