Convert excel formula into Javascript

First, you need to assign the values of all three fields to variables:

var a = this.getField("name of field A").value
var b = this.getField("name of field B").value
var c = this.getField("name of field C").value

Then for the calculation, I noticed that your first formula only considers var b to be equal to 2  and var c to be <= 35so:

if ((b == 2)&&(c <= 35)){
        //insert code here
}

you will nest the next if inside the first one:

if ((b == 2)&&(c <= 35)){
      if ((a >= 0)&&(a < 1339)) event.value = "Profil D"
      else if ((a >= 1340)&&(a < 1674)) event.value = "Profil C"
      else if ((a >= 1675)&&(a < 2344)) event.value = "Profil B"
      else if ((a >= 2345)&&(a < 3684)) event.value = "Profil A"
      else if (a >= 3685) event.value = "Profil A+"
      else event.value = ""
}
else event.value = ""

So the full code (assuming only integer will show in your fields) in the calculate script of your answer field (D9 in your example):

//Assign variables
var a = this.getField("name of field A").value
var b = this.getField("name of field B").value
var c = this.getField("name of field C").value

//Calculation
if ((b == 2)&&(c <= 35)){
      if ((a >= 0)&&(a < 1339)) event.value = "Profil D"
      else if ((a >= 1340)&&(a < 1674)) event.value = "Profil C"
      else if ((a >= 1675)&&(a < 2344)) event.value = "Profil B"
      else if ((a >= 2345)&&(a < 3684)) event.value = "Profil A"
      else if (a >= 3685) event.value = "Profil A+"
      else event.value = ""
}
else event.value = ""

I would suggest you consider learning JavaScript, or hiring an expert to help you with this conversion.

https://www.pdfscripting.com/public/main.cfm

This is very basic JS syntax. "IF" in JS is "if" and "AND" in JS is "&&".

You can learn more about it in multiple locations, such as here: https://www.w3schools.com/js/

Note that while the basic syntax is the same, though, Acrobat and web JavaScript implementations have a different set of objects, properties and methods.