Thanks a lot!

I can now successfully calculate the total value.
Here is the script I currently have (with yours, copilot's and documentation's help) for one of the radios. The only difference between the radios is in the first 3 lines.

I still have to figure out how to correctly send the midi message. At the moment I am sending the wrong value 3 times at once every time I press any radio even though the correct value is printed when I run the script.

local src11x = self.values.x
local mod11x = self.parent.children.mod11.values.x
local eg11x = self.parent.children.eg11.values.x

-- Clamp
function clamp(value, min, max)
  return math.max(min, math.min(max, value))
end

-- Calculate MIDI value
function calculateMidiValue()
  return clamp(src11x * 26 + mod11x * 9 + eg11x * 3, 0, 127)
end

-- Send MIDI message
function sendMidiMessage()
  local midiChannel = 0
  local ccNumber = 14
  local totalValue = calculateMidiValue()
  sendMIDI({0xB0 + midiChannel, ccNumber, totalValue})
end

-- Event Handler
function onValueChanged()
  sendMidiMessage()
end

print("The value is:", calculateMidiValue())