IFERROR and Multiple IF Statements Not Returning Expected Result

hi eforsadoko, welcome to the forum. first question; why do you need to range up to Column V when you only need the 11th column?
=IFERROR(VLOOKUP(B2,NewData!A:V,11,FALSE),"NA")

text in excel are considered to be bigger than numbers. you can try somewhere:
="ABC">0

so your first test of lesser or equals to 100 will fail. a text is greater than a number. it goes to the 2nd test where you put whether is it greater than 300. it is, so it returns "3". put it this way instead:
=IF(C2="NA","NA",IF(C2<=100,1,IF(C2>300,2,100)))

so only when it fails the first test, it will continue with the rest. also, you don't need double quotes for numbers:
=IF(C2<=100,"1",IF(C2>300,"3",IF(C2>100,"2","NA")))