Adding 3 linked lists in the correct order and returning result linked list

#define E_INVALID_PARAMETER -1

int getNumber(node* root)
{
if(root == NULL)
{
return E_INVALID_PARAMETER;
}

int ret = 0;
node* temp = root;
while(temp)
{
int data = temp->data;
//checks to find data is valid
ret = ret*10 + data;
temp = temp->next;
}
return ret;
}

node * createLL(int num)
{
node* head = null;
node* prev = null;

while(num)
{
int data = num % 10;
head = (node *)malloc(sizeof(node));
if(head == NULL)
{
return NULL;
}
head->data = data;
head->next = prev;
prev = head;
num = num/10;
}

return head;
}

node * sumList (node* a, node *b, node* c)
{
if(a == NULL || b == NULL || c == NULL)
{
return NULL;
}
int aNum = 0;
int bNum = 0;
int cNum = 0;

aNum = getNumber(a);
bNum = getNumber(b);
cNum = getNumber(c);

if(a == E_INVALID_PARAMETER || b == E_INVALID_PARAMETER || c == E_INVALID_PARAMETER)
{
return NULL;
}

return createLL(aNum + bNum + cNum);
}