CareerCup

The easiest way would be to use the library function replace() function but since the question specifically asks not to use any library functions, the alternative way is to use three pointers, prev, curr, and next. If the three pointers match any one of the special characters below, then use the pop() function to modify the string in place.
If you're using python, another quick way to do this would be to use the python's slicing to find the strings and replace it with the appropriate character to be replaced. I present both solutions below.

Solution:

def decodeString(encodedStr):
    curr = 0
    encodedStr = list(encodedStr)
    specialCharacters = \
    {
        '%20':' ',
        '%3A':'?',
        '%3D':':'
    }
    while curr < len(encodedStr) - 2:
        specialCharacter = ''.join(encodedStr[curr:curr+3])
        if specialCharacter in specialCharacters:
            encodedStr.pop(curr+2)
            encodedStr.pop(curr+1)
            # Replace by the appropriate decoded character
            encodedStr[curr] = specialCharacters[specialCharacter]
        else:
            curr += 1
    return ''.join(encodedStr)

def decodeUsingThreePointers(encodedStr):
    prev, curr, next = 0, 1, 2
    encodedStr = list(encodedStr)
    while prev < len(encodedStr) - 2:
        if encodedStr[prev] == '%' and encodedStr[curr] == '2' and encodedStr[next] == '0':
            encodedStr.pop(next)
            encodedStr.pop(curr)
            encodedStr[prev] = ' '
        elif encodedStr[prev] == '%' and encodedStr[curr] == '3' and encodedStr[next] == 'A':
            encodedStr.pop(next)
            encodedStr.pop(curr)
            encodedStr[prev] = '?'
        elif encodedStr[prev] == '%' and encodedStr[curr] == '3' and encodedStr[next] == 'D':
            encodedStr.pop(next)
            encodedStr.pop(curr)
            encodedStr[prev] = ':'
        else:
            prev += 1
            curr += 1
            next += 1
    return ''.join(encodedStr)

print(decodeUsingThreePointers('kitten%20pic.jpg')) # kitten pic.jpg
print(decodeUsingThreePointers('dog%3Aimg.jpg')) #dog?img.jpg
print(decodeUsingThreePointers('lion%3Dbook.txt')) #lion:book.txt
print(decodeString('kitten%20pic.jpg')) #kitten pic.jpg
print(decodeString('dog%3Aimg.jpg')) #dog?img.jpg
print(decodeString('lion%3Dbook.txt')) #lion:book.txt

// JDK 9 only
private static final Map<String, Character> SPECIAL_CHARS = Map.of("20", ' ', "3A", '?', "3D", ':');

public static void decode(char[] str) {
    int readIdx = 0, writeIdx = 0;

    while (readIdx < str.length) {
        char replacement = str[readIdx];

        if (replacement == '%' && readIdx + 2 < str.length) {
            Character c = SPECIAL_CHARS.get("" + str[readIdx + 1] + str[readIdx + 2]);
            if (c != null) {
                replacement = c;
                readIdx += 2;
            }
        }

        str[writeIdx++] = replacement;
        readIdx++;
    }

    while (writeIdx < str.length) {
        str[writeIdx++] = '\u0000';
    }
}

#define STR_LEN 16

void decode(char str[STR_LEN])
{
    int i, j;
    
    j = 0;
    while('%' != str[j])
        j++;

    i = j;
    j += 2;

    if ('0' == str[j])
        str[i] = ' ';
    if ('A' == str[j])
        str[i] = '?';
    if ('D' == str[j])
        str[i] = ':';

    while(STR_LEN > j) {
        i++;
        j++;
        str[i] = str[j];
    }

    str[++i] = '\0';

    printf("modified string : %s\n", str);
    return;
}


int main()
{
    char str[STR_LEN] = {'k','i','t','t','e','n','%','3','D','p','i','c','.','j','p','g'};

    decode(str);

    return 0;
}

In Javascript

{{ function strMap(ori, rep){
this.original = ori;
this.replacement = rep;
}

let strMapArr = [];
strMapArr.push(new strMap("%20"," "));
strMapArr.push(new strMap("%3A","?"));
strMapArr.push(new strMap("%3D",":"));

function replaceSpecail(str){
let found = false;
let key="";
let result="";
for(i=0;i<str.length;i++){
if (str[i] === '%') {
key= str[i]+str[i+1]+str[i+2];
for(n=0;n<strMapArr.length;n++){
if (key === strMapArr[n].original){
result += strMapArr[n].replacement;
i = i + 2;
}
}
key="";
} else
result += str[i];
}
return result;
}

console.log(replaceSpecail('kitten%20pic.jpg'));
console.log(replaceSpecail('dog%3Aimg.jpg'));
console.log(replaceSpecail('lion%3Dbook.txt'));
}}

def encodeDecode(str):

charDict = {
'%20': ' ',
'%3A': '?',
'%3D': ':'
}
s1 = str
length = len(s1)
ds = ""
i =0

while i < length:
if s1[i] == '%' and i < length:
popString = s1[i]+s1[i+1] + s1[i+2]
for key in charDict:
if key == popString:
popString = charDict[key]
ds = ds+popString
i += 3
elif i > length:
break
else:
ds += s1[i]
i += 1
print ds

encodeDecode("abc%20pic.jpg%3Aabc%3Dvv")
encodeDecode("xy%3Apic.jpg%20abc%3Dhh")

def encodeDecode(str):

    charDict = {
        '%20': ' ',
        '%3A': '?',
        '%3D': ':'
    }
    s1 = str
    length = len(s1)
    ds = ""
    i =0

    while i < length:
        if s1[i] == '%' and i < length:
            popString = s1[i]+s1[i+1] + s1[i+2]
            for key in charDict:
                if key == popString:
                    popString = charDict[key]
                    ds = ds+popString
            i += 3
        elif i > length:
            break
        else:
            ds += s1[i]
            i += 1
    print ds

encodeDecode("abc%20pic.jpg%3Aabc%3Dvv")
encodeDecode("xy%3Apic.jpg%20abc%3Dhh")

def encodeDecode(str):

    charDict = {
        '%20': ' ',
        '%3A': '?',
        '%3D': ':'
    }
    s1 = str
    length = len(s1)
    ds = ""
    i =0

    while i < length:
        if s1[i] == '%' and i < length:
            popString = s1[i]+s1[i+1] + s1[i+2]
            for key in charDict:
                if key == popString:
                    popString = charDict[key]
                    ds = ds+popString
            i += 3
        elif i > length:
            break
        else:
            ds += s1[i]
            i += 1
    print ds

encodeDecode("abc%20pic.jpg%3Aabc%3Dvv")
encodeDecode("xy%3Apic.jpg%20abc%3Dhh")