Dynamically Generating Plot with scatter3d and Shiny

I'm trying to create a quick app that lets a user select 3 variables and regenerates a 3D scatter with scatter3D. I keep hitting this error when using shiny and I can't see to rectify it.

Error: not all arguments have the same length

My code also works if replace:

x = paste("df.output$",input$test,sep=""),
y = paste("df.output$",input$test2,sep=""),
z = paste("df.output$",input$test3,sep=""),

with

x = df.output$age_scaled
y = df.output$freq_scaled
z = df.output$bonus_scaled

My ui function looks like this

ui <- fluidPage(
  titlePanel("3 Dimensional Cluster Analysis"),
  sidebarLayout( 
  sidebarPanel(
  selectInput("test", "X-Axis", choices=colnames(df.output) , 
  selected=colnames(df.output[1]), width = NULL, size = NULL),
    selectInput("test2", "Y-Axis", choices=colnames(df.output), 
  selected=colnames(df.output[2]), width = NULL, size = NULL),
    selectInput("test3", "Z-Axis", choices=colnames(df.output), 
  selected=colnames(df.output[3]), width = NULL, size = NULL)),

   mainPanel(
    rglwidgetOutput("plot",  width = 1000, height = 500)
      )
    ))

Server function looks like this

library(rgl)

server <- (function(input, output) 
{
  # reactive({
  #   a <- paste("df.output$",test$input,sep="")
  # })
  output$plot <- renderRglwidget(
    {
      rgl.open(useNULL=T)
      scatter3d(
        x = paste("df.output$",input$test,sep=""),
        y = paste("df.output$",input$test2,sep=""),
        z = paste("df.output$",input$test3,sep=""),
        groups = as.factor(df.output$Cluster), 
        grid=FALSE,
        surface=FALSE,
        ellipsoid=TRUE,
        ellipsoid.alpha=0.5,
        fit=smooth,
        xlab=input$test,
        ylab=input$test2,
        zlab=input$test3
      )
       par3d(mouseMode = "trackball")
       rglwidget() 
     })
})   

Your code

x = paste("df.output$",input$test,sep="")

sets x to a length 1 character vector. If you want to select a component from a dataframe, use

x = df.output[[input$test]]

Your code also doesn't use the package containing scatter3d (it's not an rgl function). There's a function with that name in the car package, and a similar name in the plot3D package.